Cheesy Broccoli-and-Rice Casserole

INGREDIENTS

6 tbsp

unsalted butter, divided

1 cup

panko (Japanese breadcrumbs)

2 cups

shredded extra-sharp Cheddar cheese, divided

3 cups

reduced-sodium, fat-free chicken broth

2 cups

milk

bay leaf

fresh thyme sprig

2 cups

chopped onion

1/2 cup

diced celery

1 8 ounce package

sliced cremini mushrooms

1 tsp

kosher salt, divided

freshly ground black pepper

ground red pepper

garlic cloves, minced

1/4 cup

all-purpose flour

1 1/2 cups

uncooked long-grain rice

1/2 cup

sour cream

1/2 cup

mayonnaise

1/8 tsp

freshly grated nutmeg

3 cups

fresh broccoli florets (about 2 heads)