1 head cauliflower, cored, broken into bite-sized florets
2 medium-small leeks, cleaned, halved lengthwise, sliced into 1/4-in thick slices, white and light green parts only, about 2 cups
2 Tbsp butter
2 Tbsp all purpose flour
1 1/3 cups milk
4 ounces Gruyere cheese, grated
1 teaspoon minced fresh thyme or 1/2 teaspoon dried thyme
1/8 teaspoon grated nutmeg
1 teaspoon salt
Pepper

Topping:
1 1/2 cups fresh breadcrumbs (3 slices of sandwich bread or rustic bread, pulsed in a food processor)
1 Tbsp butter
1/4 teaspoon salt
Pepper