Cauliflower-and-Mushroom Roast

head cauliflower (about 3 pounds), trimmed and cut into 1-inch slices

extra-virgin olive oil

shiitake mushrooms, stemmed, halved if large

cremini mushrooms, trimmed and cut into 1/4-inch slices

slices rustic bread (each 1 inch thick), crusts removed, torn into small pieces (1 1/2 cups)

Kosher salt and freshly ground pepper

Gruyere or fontina (or a combination), thinly sliced