Preheat oven to 425°F. Coat a 3-quart broiler-safe casserole dish with 1 teaspoon butter.
Melt the remaining 4 tablespoons butter in a large pot over medium-high heat. When the foaming subsides, reduce heat to medium; add garlic, ½ teaspoon salt and ¼ 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.
Bake until the cauliflower is tender and beginning to brown, 35 to 40 minutes. Remove from oven.
Position a rack in upper third of oven; preheat broiler to high.
Combine cheese, sesame seeds, poppy seeds, caraway seeds and the remaining ½ teaspoon salt and ¼ 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.
To make ahead: Prepare through Step 3 up to 1 hour ahead; finish with Steps 4-5 just before serving.
158 calories;10 g fat(5 g sat); 2 g fiber; 11 g carbohydrates; 7 g protein; 51 mcg folate; 25 mg cholesterol; 4 g sugars; 0 g added sugars; 359 IU vitamin A; 29 mg vitamin C; 196 mg calcium; 1 mg iron; 366 mg sodium; 305 mg potassium
Vitamin C (48% daily value), Calcium (20% dv)