Instructions

1. Cut 2 1/2 pounds cauliflower into rough 1-inch florets. Place in a large microwave-safe bowl and cover with a damp paper towel. Microwave on HIGH until the cauliflower is fork tender but not mushy, 5 to 7 minutes. Transfer to a 9x13-inch baking dish. Season with 2 teaspoons of the kosher salt and 1/2 teaspoon of the black pepper and toss to combine. Arrange into an even layer.
2. Arrange a rack in the middle of the oven and heat the oven to 450ºF. Meanwhile, dice 1/2 large yellow onion until you have 1 cup. Thinly slice 6 large garlic cloves. Grate 6 ounces Gruyère cheese and 6 ounces sharp cheddar cheese on the large holes of a box grater (about 1 1/2 cups each).
3. Melt 5 tablespoons of the unsalted butter in a medium saucepan over medium-high heat. Add the onion, garlic, 3 fresh thyme sprigs, and remaining 1 teaspoon kosher salt. Cook, stirring occasionally, until the onion and garlic are softened and translucent, 5 to 7 minutes. Add 1 tablespoon Dijon mustard and cook for 30 seconds. Sprinkle with 1/2 cup all-purpose flour. Cook, stirring constantly, until the flour is slightly toasted, about 30 seconds.
4. While whisking constantly, slowly pour in 3 cups whole milk. Add the Gruyère, cheddar, and remaining 1/2 teaspoon black pepper. Stir and cook until the cheese is melted and the sauce is bubbling, about 5 minutes. Remove and discard the thyme sprigs. Pour the sauce evenly over the cauliflower.
5. Place the remaining 3 tablespoons unsalted butter in a small microwave-safe bowl. Microwave on HIGH until melted, about 30 seconds. Add 1 cup panko breadcrumbs and season with a pinch each of kosher salt and black pepper. Stir with a fork or mix with your fingertips until the panko is evenly moistened. Sprinkle evenly over the cauliflower.
6. Bake until the sauce is bubbling and the panko is golden brown, 10 to 15 minutes. If you’d like a browner top, broil for 2 to 3 minutes more. Let cool for 10 minutes to let the sauce thicken before serving.