With a vegetable peeler or paring knife peel squash. Cut squash and onion into 1/4-inch dice and cut broccoli rabe into 1/2-inch pieces. In a 12-inch nonstick skillet heat oil over moderately high heat until hot but not smoking and add vegetables and salt to taste, stirring to coat with oil. Add water and cook vegetables, covered, over moderate heat 5 minutes, or until tender. Remove lid and cook vegetables over moderately high heat, stirring occasionally, until liquid is evaporated and vegetables begin to brown, about 3 minutes more.