Sourdough Au Gratin Potatoes

Creamy, cheesy, and packed with garlic flavor, these sourdough au gratin potatoes are made with a rich homemade cheese sauce thickened with sourdough discard. They're the perfect side dish for holiday dinners, Sunday roasts, or whenever you're craving comfort food.

Course Side Dish

Cuisine French

Keyword Sourdough Discard

Prep Time 25

Cook Time 1 15

Total Time 1 40

Servings Servings

Calories 548kcal

Author Kate Freebairn

Equipment

Baking Dish 9 x 13-inch
Food Processor or mandoline, or sharp knife
Large Mixing Bowl
Medium Saucepan

Ingredients

50 g Butter
3 Garlic Cloves (finely chopped)
120 g Sourdough Starter (or discard)
650 g Heavy Cream (or half-and-half or whole milk)
230 g Cheddar Cheese (shredded)
120 g Gruyere Cheese (shredded)
3 sprigs Fresh Thyme
5 g Salt
Black Pepper (freshly ground, to taste)
1.75 kg Potatoes (approx. 4 lbs, thinly sliced)

Instructions

Preheat the oven to 375℉/180℃ and grease a 9x13-in casserole dish.
In a medium saucepan, melt the butter over medium heat. Add the chopped garlic and cook until the garlic is fragrant and softening, 1 to 2 minutes. Mix in the sourdough starter and cook until it turns a rich golden brown.
Slowly pour in the cream, whisk to combine, and cook until the cream sauce is warm and thick. Stir in ¾ of the cheddar and Gruyere as well as the thyme leaves, salt, and a few grinds of black pepper. Stir until most of the cheese has melted and the sauce is fairly smooth.
Next you’ll combine the potatoes and the sauce, ensuring all of the potato slices are evenly coated.
Carefully transfer the cheesy potatoes to the prepared baking dish. Try to keep the potatoes fairly even on top so the dish cooks evenly. Cover the dish with aluminum foil with a few holes in the top for steam to escape. Bake for 60 to 75 minutes, or until the potatoes are fork-tender all the way through.
Remove the foil and scatter the remaining cheddar and Gruyere over the top of the potatoes. Bake for about 10 to 15 minutes longer, or until the top is golden and bubbling and smells incredible. Let the dish stand a few minutes before serving.

Notes

I don’t peel my potatoes because I like the look and texture of the browned edges. Plus peeling them is a pain, but feel free to peel your potatoes if you'd like.
I prefer the texture of thinly sliced potatoes, so if you don’t have or want to use the food processor, you could also use a mandoline or a sharp knife.
If your potatoes are going to sit out for a little while, soak them in a large bowl of cold water to prevent discoloration.

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