How to Make It
Preheat oven to 350°. Melt 2 Tbsp. butter. Combine melted butter with panko and 1 cup shredded Cheddar cheese; toss to coat.
Bring broth and next 3 ingredients to a simmer in a medium saucepan over medium-low heat. Reduce heat to low; cover and keep mixture warm until ready to use.
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 1/2 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.)
Remove and discard bay leaf and thyme from broth mixture. Gradually stir warm broth mixture into mushroom mixture. Add remaining 1 cup cheese, and stir until well blended and smooth. Stir in rice and next 3 ingredients. Cover Dutch oven with a tight-fitting lid.
Bake at 350° for 25 to 30 minutes or until rice is tender and liquid is absorbed. Remove Dutch oven from oven; increase oven temperature to broil.
Microwave broccoli, 1/4 cup water, and remaining 1/2 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. Stir broccoli into rice mixture in Dutch oven. If desired, transfer mixture to a broiler-safe serving dish coated lightly with cooking spray.
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.