I love a rich, comforting au gratin for holidays or any time you want a hearty side, but many of us are choosing cleaner, dairy-free options. This Whole30/Paleo Potato-Leek Au Gratin keeps the cozy, indulgent texture of traditional gratin while replacing dairy with flavorful, compliant ingredients. Thinly sliced potatoes layered with sautéed leeks and onions, then baked in a savory coconut-chicken broth sauce—this version proves you don’t need cheese to have a luxurious side dish.
Potatoes are sliced paper-thin and combined with a fragrant sauté of leeks, onions, and garlic. The creamy sauce uses canned full-fat coconut milk thickened slightly with arrowroot and enriched with ghee and chicken broth for depth. When baked under foil and finished uncovered (or briefly under the broiler), the top develops a beautiful golden finish and the interior becomes tender and saucy. Letting the gratin rest before serving allows the sauce to set so every slice holds together cleanly.
If you don’t already own a mandolin, it’s worth getting one before the holidays because it makes quick work of thin, even potato slices. A mandolin ensures uniform cooking and a pleasing texture without long prep work. If you prefer not to buy one, use a sharp knife and take care to slice as evenly as possible.
This recipe serves eight and makes an excellent accompaniment to roasted proteins, a holiday roast, or a simple roasted vegetable plate. Below is the full ingredient list and step-by-step instructions rewritten for clarity and ease of use.
- 3 lbs. large yellow potatoes
- 1/2 large yellow onion, sliced very thinly
- 2 leeks, white and light green parts, cleaned and sliced thinly
- 2 cloves garlic, minced
- 2 tbsp. olive oil
- 1 tbsp. ghee
- cooking spray
- 2 tbsp. ghee
- 2 cloves garlic, minced
- 1 cup full-fat canned unsweetened coconut milk (blend first if separated)
- 1 cup chicken broth
- 1 tbsp. arrowroot starch
- salt and pepper
- 4 sprigs fresh thyme, leaves removed
- aluminum foil
- Preheat the oven to 350°F (175°C).
- Peel the potatoes. Using a mandolin or a very sharp knife, slice the potatoes about 1/8 inch thick. Arrange the slices in a large bowl and set aside.
- In a skillet over medium-high heat, warm 2 tbsp olive oil and 1 tbsp ghee. Add the sliced leeks, sliced onion, and 2 cloves minced garlic. Season with salt and pepper and sauté until the onions and leeks are very tender, about 5–7 minutes. Remove from heat.
- Toss the sautéed onion and leek mixture with the sliced potatoes in the large bowl so the slices are evenly coated. Season again with salt and pepper to taste.
- Lightly spray a casserole dish with cooking spray. Spread the potato and onion mixture evenly in the dish, arranging slices uniformly for even cooking.
- To make the sauce, melt 2 tbsp ghee in a skillet over medium heat. Add 2 minced garlic cloves and cook briefly (30 seconds to 1 minute) until fragrant, taking care not to burn. In a small bowl, whisk the chicken broth together with 1 tbsp arrowroot starch until smooth. Pour the broth mixture into the skillet, add 1 cup coconut milk, and whisk to combine.
- Add the fresh thyme leaves and season with salt and pepper. Let the sauce simmer gently for about 5 minutes, stirring regularly, until it thickens slightly.
- Pour the warm sauce evenly over the potatoes in the casserole. Seal the dish tightly with aluminum foil and bake at 350°F for 40 minutes. Remove the foil and bake uncovered for another 15 minutes. If the top is not golden, finish briefly under the broiler for 3–5 minutes while watching closely to avoid burning.
- Remove from the oven and allow the gratin to rest for 10–15 minutes before serving. This rest time lets the sauce thicken so the slices hold together when plated.
- Serve warm and enjoy.
Notes and tips: For best results, slice the potatoes as uniformly as possible so they cook evenly. If the coconut milk separates in the can, blend it briefly until smooth before measuring. Arrowroot provides a glossy, neutral-thickened sauce; if you prefer a slightly thicker sauce, you can increase arrowroot by a small amount, mixing it thoroughly with the broth first to avoid lumps. Ghee adds richness while keeping the recipe compliant for many dairy-free diets; you can substitute olive oil if necessary.
Storage: Leftovers keep well refrigerated for up to 3 days. Reheat gently in a covered dish at low oven temperature or in a skillet with a splash of broth to prevent drying. This gratin also holds up well for make-ahead preparation—assemble and refrigerate (covered) until ready to bake, then add a few extra minutes to the bake time if starting cold.
This Potato-Leek Au Gratin is a comforting, crowd-pleasing side that works with a variety of main dishes and fits Whole30 and Paleo preferences without sacrificing texture or flavor. Enjoy it as part of a holiday spread or a cozy weeknight meal.