Step 1

Preheat oven to 425 degrees F. Coat a 3-quart broiler-safe casserole dish with 1 teaspoon butter.

Step 2

Melt the remaining 4 tablespoons butter in a large pot over medium-high heat. When the foaming subsides, reduce heat to medium; add garlic, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook, stirring, until fragrant, 1 to 2 minutes. Add flour, whisk to combine, and cook, stirring constantly, until no flour smell remains, 2 to 3 minutes. Gradually whisk in milk, whisking between additions, until the mixture is smooth. Bring to a lively simmer. Cook, whisking, until thickened, about 2 minutes. Add cauliflower, 3 tablespoons chives and cayenne (if using), folding to combine. Transfer to the prepared pan and smooth the top.

Step 3

Bake until the cauliflower is tender and beginning to brown, 35 to 40 minutes. Remove from oven.

Step 4

Position a rack in upper third of oven; preheat broiler to high.

Step 5

Combine cheese, sesame seeds, poppy seeds, caraway seeds and the remaining 1/2 teaspoon salt and 1/4 teaspoon pepper in a medium bowl. Sprinkle on the casserole. Broil until the cheese is melted and the top is golden, about 2 minutes. Serve topped with the remaining 1 tablespoon chives.