Bring a large pan of salted water to a boil. Break the cauliflower into small florets and cook for 3 minutes. Remove from the heat, drain well, and transfer to a large, shallow ovenproof dish.
Melt the butter in a small skillet over medium heat. Add the sliced mushrooms, stir to coat, and cook gently for 3 minutes. Remove from the heat and add to the cauliflower. Season with salt and pepper.
Combine the breadcrumbs, cheese, and herbs in a small mixing bowl, then sprinkle the crumbs over the vegetables. Dice the butter for the topping and dot it over the crumbs. Place the dish in a preheated oven 425 degrees and bake for 15 minutes, or until the crumbs are golden brown and crisp.