Cut 1 large head of cauliflower into ½-inch pieces. Line a baking sheet with foil and drizzle with olive oil (about 2 tablespoons). Lay cauliflower slices on baking sheet in single layer and layer with slices from 1 large onion. Top with rosemary and thyme sprigs. Drizzle with additional olive oil (about 2 more tablespoons). Salt and pepper to taste. Transfer foil with vegetables to grill. Grill, covered, about 20 minutes, turning cauliflower pieces over halfway through cooking.