In a covered saucepan, cook onions in water for 25 minutes or until tender. Drain, reserving 1/4 cup liquid. Combine onions, peas, soup, 2 tablespoons pimientos and reserved cooking liquid; stir to coat. Transfer to a greased 1-1/2-qt. baking dish. Bake, uncovered, at 350 degrees F for 35 minutes. Sprinkle with cheese and remaining pimientos. Bake 5 minutes longer or until the cheese is melted.