A lighter take on classic potato gratin that swaps heavy cream for 1% milk and vegetable broth, and replaces some potato slices with fennel for a fresh, aromatic twist.
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Potato gratin is a comforting classic, and this version lightens it up without sacrificing flavor. Instead of heavy cream, a mix of vegetable broth and low-fat milk forms the sauce, thickened with a simple roux. Slices of fennel are added alongside potatoes for a subtle anise note and extra texture.
🧅 Main Ingredients + Notes
Small shortcuts and prep steps make weeknight cooking easier: grate the cheeses ahead of time, use sautéed onions if you don’t have leek confit, and slice vegetables uniformly for even cooking.
For full measurements and steps, see the recipe card below.
- Potatoes – Russet work well; Yukon Gold is a fine alternative.
- Fennel bulb – sliced thin for a mild, aromatic flavor.
- Swiss cheese – Gruyere preferred, but any Swiss-style cheese is fine.
- Parmesan cheese – for a salty, savory finish.
- Leek confit – leeks slowly cooked in butter and olive oil until soft; sautĂ©ed yellow onions may be substituted.
- Thyme – fresh or dried.
- Rosemary – fresh or dried.
- Unsalted butter – used to make the roux.
- All-purpose flour – for thickening the sauce.
- Low-fat milk – keeps this gratin lighter than the traditional cream version.
- Vegetable broth – adds depth; use a good-quality broth or paste.
Disclosure: This post may contain affiliate links to products and foods I use in my kitchen. At no extra cost to you, I may earn a commission if you click through and make a purchase.
🔪 Recommended Equipment
- Chef’s knife
- Medium skillet (for cooking leeks or onions)
- Vegetable peeler
- Hand-held mandolin (for even, thin slices)
- Medium pot or small skillet (for the roux)
- Whisk
- Cheese grater
đź“ť Instructions Overview
Full details are in the recipe card below. Here’s a concise overview to guide you:
Step 1
Make the leek confit or sauté diced onions in olive oil or butter until softened.
Step 2
Prep the potatoes and fennel. Peel the potatoes and slice potatoes and fennel thinly, ideally about 1/8″ thick for even cooking.
Step 3
Combine the sliced potatoes, fennel, cooked leeks/onions, most of the grated cheeses, and herbs in a large bowl, then transfer to a baking dish.
Step 4
Make the roux and sauce. Melt butter in a small skillet, whisk in flour to form a paste, then slowly whisk in the combined milk and vegetable broth until smooth. Bring to a gentle simmer until slightly thickened, then pour over the vegetables.
Step 5
Bake covered until the potatoes are tender. Uncover, sprinkle with remaining cheese, and bake briefly until the top is melted and lightly browned. Serve hot.
⏰ Tips to Simplify and Save Time
- Grate the cheeses and store in the fridge ahead of time.
- Swap leek confit for sautéed onions if you want to shave prep time.
- Use a mandolin for fast, even slicing; uniform slices help the gratin cook evenly.
đź’ˇ Ideas for Variations
Finish with gremolata-style breadcrumbs or plain breadcrumbs dotted with a few small pats of butter for added crunch and flavor.
🤷‍♀️ FAQ
Do I need to make a roux for this gratin?
Yes. Since low-fat milk does not thicken the way heavy cream does when baked, a roux (butter + flour) is used to create a creamy sauce when combined with the milk and vegetable broth.
Skinny Potato and Fennel Gratin
Equipment
-
Chef’s knife
-
Medium skillet
-
Hand-held mandolin
-
Box grater
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9″ x 9″ baking dish
-
Small skillet
-
Whisk
-
Aluminum foil
Ingredients
- 2 ½ large russet potatoes
- 2 bulbs fennel
- ÂĽ cup grated Gruyere cheese
- ÂĽ cup grated Parmesan cheese
- 1 cup Leek confit or 2 cups diced yellow onions
- 1 teaspoon dried rosemary or 2 tablespoons fresh
- 1 teaspoon fresh thyme leaves or 2 tablespoons fresh
- 1 teaspoon kosher salt
- Âľ teaspoon freshly ground pepper
- 2 tablespoons unsalted butter
- 3 tablespoons all-purpose flour
- 2 cups vegetable broth
- â…“ cup low-fat milk
Instructions
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Preheat the oven to 400°F (200°C).
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If not using leek confit, sauté the diced onions in 1 tablespoon olive oil or unsalted butter until softened.
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Thinly slice the potatoes and fennel with a sharp knife or mandolin. Even, thin slices (about 1/8″) cook more quickly and evenly.
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In a large bowl, combine the sliced potatoes, fennel, leek confit or cooked onions, two-thirds of the grated cheeses, thyme, rosemary, salt, and pepper. Gently toss and transfer to a baking dish.
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In a small skillet over medium heat, melt the butter. Whisk in the flour and cook 2–3 minutes, whisking occasionally, to form a roux.
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Combine the vegetable broth and milk in a measuring cup. Gradually whisk the liquid into the roux, starting with a few tablespoons at a time until it becomes smooth. Bring to a gentle simmer and cook a few minutes until slightly thickened, then pour the sauce over the vegetables in the baking dish.
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Cover the dish with foil and place it on a baking sheet. Bake for about 45 minutes, or until the potato slices are tender when pierced.
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Uncover, sprinkle the remaining cheese over the top, and bake 5–10 minutes more uncovered, until the cheese is melted and lightly golden.
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Serve piping hot.
Nutrition
| Carbohydrates: 26.5 g
| Protein: 6.9 g
| Fat: 10.3 g