Slow-Baked Potato and Onion Casserole
Pommes de terre a la boulangere is a French country classic that transforms humble potatoes and onions into a luxurious side dish through patient layering and braising. This rustic gratin uses beef stock to create a savory, deeply flavored dish where potatoes absorb rich, caramelized onion essence while their edges turn tender and their centers stay firm. Fresh thyme adds earthy complexity against sweet caramelized onions and buttery potatoes. The long, slow bake allows flavors to meld completely. Perfect for Sunday dinners, special occasions, or whenever you want an impressive vegetable dish that feels homestyle rather than fussy. What sets this version apart is its simplicity—no cream, no cheese—letting the interplay of stock, butter, and slow heat create elegance without complication.
Ingredients
- 3 tablespoon butter
- 1 tablespoon olive oil
- 4 onion, peeled and sliced
- 2 pound potato, thinly sliced
- salt(optional)
- black pepper(optional)
- fresh thyme, leaves picked(optional)
- 2 cup beef stock
Instructions
- 1
Preheat the oven to 300 degrees F
- 2
Melt half the butter with olive oil in a saute pan over medium heat
- 3
Gently fry the onions until soft and lightly golden, about 15 minutes
- 4
Spread half the cooked onions in the bottom of a casserole
- 5
Layer half the potatoes on top, season with salt and pepper, and scatter with thyme leaves
- 6
Add the remaining onions as the next layer
- 7
Top with the remaining potatoes as the final layer
- 8
Pour the beef stock over everything
- 9
Cover the casserole with foil and bake until all liquid is absorbed, 2 to 3 hours
- 10
Remove the foil for the last hour if you prefer a crisp top
Tips
Don't skip the preliminary onion fry. The 15-minute caramelization develops sweetness and depth that seasons the entire dish—rushing this step flattens the final flavor.
Slice potatoes uniformly thin so they cook evenly throughout. Inconsistent thickness creates both undercooked centers and overcooked edges.
Remove foil in the final hour only if your oven runs cool or if you want pronounced browning. Early uncovering can leave potatoes dry.
Good to Know
Cover and refrigerate up to 4 days. Reheat covered in a 325 F oven until warmed through, about 30 minutes.
Assemble through the stock step, cover, and refrigerate up to 12 hours before baking. Add 15-20 minutes to total bake time if baking straight from cold.
Serve hot from the casserole as a side to roasted meats, particularly beef, lamb, or pork. Also works alongside braised dishes or simple grilled fish.
Common Mistakes
Skip the onion fry to avoid pale, sharp-tasting dish that lacks depth.
Uncover too early to avoid dry, unevenly cooked potatoes.
Use thick-cut potatoes to avoid undercooked centers and mushy edges.
Pour cold stock over to avoid temperature shock that extends cooking time unevenly.
Substitutions
Dairy-Free Swaps
General Alternatives
FAQ
Can I make this ahead?
Yes. Assemble through the stock step, cover, and refrigerate up to 12 hours. Bake directly from cold, adding 15-20 minutes to cooking time. You can also bake fully, cool, refrigerate up to 4 days, then reheat covered at 325 F for about 30 minutes.
What if I don't have beef stock?
Chicken stock works well and gives a lighter flavor. Vegetable stock works for vegetarian versions but produces less savory depth. Bone broth intensifies umami. Use equal quantities as the recipe calls for beef stock.
How do I get a crisp top?
Uncover the casserole for the final hour of baking so the top layer of potatoes browns and crisps in direct oven heat. If the top browns too quickly before potatoes are tender inside, recover loosely with foil and continue baking.