Step 1

Preheat oven to 350°F.

Step 2

Melt 2 tablespoons butter in a Dutch oven over medium heat.

Step 3

Add onion and garlic; cook, stirring occasionally, until softened, about 5 minutes.

Step 4

Add another 4 tablespoons butter; let met.

Step 5

Stir flour into butter and onions. Cook, stirring, 2 minutes. Stir in stock and milk until combined.

Step 6

Cook, stirring oven, until mixture comes to a simmer and has slightly thickened, about 5 minutes.

Step 7

Remove from heat. Stir in mustard and 1 ½ cups cheese until smooth. Gently stir in broccoli and rice.

Step 8

Taste for seasonings; add salt ¼ teaspoon at a time to reach desired flavor. Stir in pepper.

Step 9

Transfer to a 13x9-inch casserole dish.

Step 10

In a medium bowl, stir together breadcrumbs and remaining 2 tablespoons butter.

Step 11

Sprinkle breadcrumbs over casserole; top with remaining 1 cup cheese.

Step 12

Bake until golden and bubbly, about 30 minutes.