Make casserole topping: Preheat oven to 350°F. Melt 2 Tbsp. butter. Combine melted butter with panko and 1 cup shredded Cheddar cheese; toss to coat.

Make broth: Bring broth and next 3 ingredients (through thyme sprig) to a simmer in a medium saucepan over medium-low heat. Reduce heat to low; cover and keep mixture warm until ready to use.

Cook vegetables: Melt remaining 4 Tbsp. butter in a large shallow Dutch oven or ovenproof skillet over medium heat. Add onion and celery to Dutch oven, and cook, stirring occasionally, 3 to 5 minutes or until onion is lightly browned. Add mushrooms to Dutch oven, and stir in ½ tsp. kosher salt and a pinch each of black pepper and ground red pepper. Cook, stirring occasionally, 3 to 5 minutes or until mushrooms are tender. Add garlic, and cook, stirring constantly, 45 seconds. Stir in flour until combined. (Mixture will be dry.)

Add stock mixture to vegetables: Remove and discard bay leaf and thyme from broth mixture. Gradually stir warm broth mixture into mushroom mixture.

Add cheese: Add remaining 1 cup cheese, and stir until well blended and smooth.

Add rice, sour cream, mayo, and seasoning: Stir in rice and next 3 ingredients (through nutmeg). Cover Dutch oven with a tight-fitting lid.

Bake casserole: Bake at 350°F for 25 to 30 minutes or until rice is tender and liquid is absorbed. Remove Dutch oven from oven; increase oven temperature to broil.

Cook broccoli: Microwave broccoli, ¼ cup water, and remaining ½ tsp. kosher salt in a covered microwave-safe bowl at HIGH about 2 minutes or just until broccoli is tender and bright green. Drain and pat broccoli dry.

Add broccoli to casserole: Stir broccoli into rice mixture in Dutch oven. If desired, transfer mixture to a broiler-safe serving dish coated lightly with cooking spray.

Add breadcrumbs, and broil: Sprinkle breadcrumb mixture over broccoli mixture. Place on middle oven rack, and broil 2 to 3 minutes or until topping is golden brown. Let casserole stand 5 minutes before serving.