Momma's Love Casserole

Ingredients

• 6 tablespoons unsalted butter, divided
• 1 cup panko breadcrumbs
• 2 cups extra-sharp cheddar cheese, shredded and divided
• 3 cups reduced-sodium fat-free chicken broth
• 2 cups milk
• 1 bay leaf
• 1 fresh thyme sprig
• 2 cups onion, chopped
• 1/2 cup celery, diced
• 1 (8-ounce) package sliced cremini mushrooms
• 1 teaspoon kosher salt, divided
• black pepper, to taste
• ground red pepper, to taste
• 2 cloves garlic, minced
• 1/4 cup all-purpose flour
• 1 1/2 cups long-grain rice
• 1/2 cup sour cream
• 1/2 cup mayonnaise
• 1/8 teaspoon nutmeg, freshly grated
• 3 cups fresh broccoli florets
• 1/4 cup water