Turn grill on. Line a baking sheet with foil. Drizzle some olive oil on top of foil. Lay cauliflower slices on baking sheet in a single layer. Layer onion slices on top. Place thyme and rosemary sprigs on top. Drizzle with remaining olive oil. Season to taste with salt and pepper.

Transfer foil with cauliflower to grill. Grill, covered, about 20 minutes, turning cauliflower pieces over halfway through cooking.