1 (2½ pound-head)
cauliflower, cut into florets or 1 (24 ounce bag cauliflower florets)
3 tablespoons
extra-virgin olive oil
1 teaspoon
sea salt
5 tablespoons
salted butter
½ cup
panko breadcrumbs
1
large shallot, thinly sliced
2
garlic cloves, minced
3 tablespoons
all-purpose flour
1¾ cups
whole milk
2 teaspoons
mustard powder
½ teaspoon
freshly cracked black pepper
¼ teaspoon
ground nutmeg
¾ cup
freshly grated Parmesan cheese, plus more for serving
1 cup
freshly grated Gruyère
1 tablespoon
minced fresh flat leaf parsley, for serving (optional)
Method
Preheat the oven to 400°F with a rack in the center position.
Place the cauliflower florets in a 9 x 13-inch baking dish and toss with 2 tablespoons of the olive oil and ½ teaspoon of the salt. Roast for 10 minutes.
Meanwhile, melt 2 tablespoons of the butter in a medium skillet over medium heat. Add the panko and toss until coated. Cook, stirring occasionally, until panko is golden brown, 3-4 mins. Transfer to a small bowl and wipe out the skillet.
In the same skillet, heat the remaining 1 tablespoon of olive oil over medium heat. Once the oil is glistening, add the shallot and cook, stirring, until softened, about 3 minutes. Add the garlic and cook, stirring, until fragrant, about 1 minute more. Add the remaining 3 tablespoons of butter. Once the butter is melted, sprinkle with the flour and whisk until absorbed, about 1 minute. Slowly whisk in the milk and cook, whisking, until the sauce has thickened, 4 to 5 minutes. Add the mustard powder, remaining ½ teaspoon of the salt, pepper, nutmeg, ½ cup of the Parmesan, and ¾ cup of the Gruyère. Cook, whisking often, until the cheese has melted, 2 to 3 minutes. Remove the skillet from the heat.
Once the cauliflower has roasted for 10 minutes, remove the baking dish from the oven. Pour the cream mixture over the cauliflower, then sprinkle evenly with the remaining ¼ cup Parmesan and ¼ cup Gruyère. Return to the oven and bake until the sauce is bubbling and the cheese is lightly browned, about 25-30 minutes. Remove from the oven and let cool for about 10 minutes.
Top the gratin with the toasted panko and parsley, if using, before serving.