Remove the outer green leaves from the cauliflower and cut the head into florets, discarding the stems. Place the florets on a sheet pan. Drizzle with the olive oil and sprinkle generously with salt and pepper to taste. Toss well.
Bake for 30 minutes, tossing once, until the cauliflower is tender and starts to brown.
Sprinkle with the Parmesan and Gruyere and bake for 1 to 2 more minutes, just until the cheese melts. Season to taste and serve hot or warm.