Cut all the fish and veggies into similar-sized pieces. Marinate the fish and vegetables in the marinade for at least an hour in the refrigerator. To make the marinade, purée the 2 Tbsp chopped onion, 1 Tbsp chopped fresh rosemary, 2 garlic cloves, 1 Tsp salt, 1/2 Tsp black pepper in a food processor. Drizzle in 1/2 cup olive oil while puréeing, continue to purée until smooth, about 1-2 minutes. Grill on high, direct heat: Prepare the grill for high, direct heat. Clean the grates and wipe them down with a paper towel that has been dipped in vegetable oil. Lay the skewers on the grill. Don't move them until the fish pieces are well browned on one side, about 3-6 minutes. Serve hot or at room temperature. Drizzle with lemon juice or serve with lemon wedges.