Saute onion in olive oil in a nonstick ovenproof skillet over medium heat. Add cilantro and 3 sliced squash blossoms. Cook for 1 minute. Add eggs. Season with salt and pepper. Bake at 375 degrees until just set, about 10 minutes. Top with mozzarella and 3 squash blossoms. Return to oven until cheese melts and frittata is set, about 5 minutes more.