Preheat oven to 375°. Cook pasta according to package directions.
Meanwhile, in a large skillet, brown sausage over medium heat 5 minutes; drain. In a large bowl, combine milk and soup. Stir in ½ cup onions, ½ cup cheese, peas and sausage. Drain pasta; stir into sausage mixture.
Transfer to a greased 13x9-in. baking dish. Cover and bake until bubbly, 25-30 minutes. Sprinkle with remaining onions and cheese. Bake, uncovered, until cheese is melted, 3-5 minutes longer.
