- 5 cups cauliflower florets (from 1 medium cauliflower, cored and trimmed)
- 2 Tbsp olive oil
- 3/4 tsp sea salt
- 1/2 tsp freshly ground pepper
- 4 Tbsp unsalted butter, divided
- 1/4 cup diced yellow onion
- 1/2 cup heavy cream
- 2/3 cup sour cream
- 3/4 tsp garlic powder
- 4 oz (1-¼ cup) shredded Gruyere cheese, divided
- 2/3 cup finely grated Parmesan cheese, divided
- 1/2 cup sliced almonds
- 2 scallions, sliced very thin
- Preheat the oven to 375°F (190°C). Place the cauliflower florets on a sheet pan. Drizzle with 1 tablespoon of the olive oil and sprinkle with 3/4 teaspoon salt and 1/2 teaspoon pepper. Toss well, arrange in one layer, then bake, tossing halfway through, until the cauliflower is tender and starts to brown, about 25 minutes.
- Meanwhile, heat a medium saucepan over medium heat. Add the remaining 1 tablespoon olive oil and 1 tablespoon of the butter. When the butter has melted, add the onion and cook until they’re softened and translucent, 2 to 3 minutes. Stir in the heavy cream and sour cream, then continue stirring while the mixture bubbles and thickens to the consistency of a milkshake, 2 to 3 minutes. Stir in the garlic powder and 3/4 teaspoon salt, 1/4 teaspoon pepper, and half of the Gruyere and half of the Parmesan cheese. Remove the cheese sauce from the heat and set aside.
- Grease a small (2-quart) casserole dish with 1 tablespoon of the butter. Spread the roasted cauliflower into the prepared dish, then pour the cheese sauce over the cauliflower.
- Make the topping: In a medium bowl, combine the remaining Gruyere and Parmesan cheese and the almonds and green onions. Melt the remaining 2 tablespoons of butter and pour it over the cheese-almond mixture, then toss to combine. Evenly distribute the topping over the cauliflower.
- Cover the gratin with aluminum foil and bake for 15 minutes. Remove and uncover the gratin, turn on the broiler and set a rack about 5 to 6 inches from the heat source, then broil the gratin until golden and bubbling, 1 to 2 minutes. Remove and serve.