We like to think of this recipe as a deconstructed, healthier grilled cheese. Cauliflower stands in as the base while the bread serves as a crunchy garnish.
Preheat oven to 450 degrees with racks in upper and lower thirds. Brush cauliflower on both sides with 4 tablespoons oil; spread in a single layer on a rimmed baking sheet. Toss mushrooms with 3 tablespoons oil; spread on a second sheet. Toss bread with remaining 1 tablespoon oil. Season all with salt and pepper.
Place cauliflower on top rack; roast 10 minutes. Add mushrooms on bottom rack; roast 15 minutes more. Flip cauliflower and mushrooms. Sprinkle bread over mushrooms; roast 10 minutes more. Remove from oven; transfer mushrooms and bread to sheet with cauliflower. Top with cheese; roast until melted and bubbly, 5 to 7 minutes more. Serve immediately.