Preheat the oven’s broiler to High and set a rack about 6 inches from the heat source.
Melt 1 tablespoon butter in a large oven-safe skillet over medium heat. Add mushrooms and cook, stirring occasionally, until they release their liquid and start to brown, 5 to 7 minutes. Add minced garlic and cook until fragrant, about 1 minute more.
Stir in cauliflower florets; season with salt, pepper, and red pepper flakes. Cook until cauliflower is slightly tender, 5 to 6 minutes, stirring occasionally.
In a small bowl, mix panko with Parmesan, fresh thyme, and remaining melted butter. Sprinkle evenly over the cauliflower-mushroom mixture in the skillet.
Place the skillet under the broiler, watching closely, until panko is golden brown and crispy, 3 to 5 minutes.
Remove from the oven and drizzle with lemon juice for brightness. Serve immediately.