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 1/2 cup onions, 1/2 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. Freeze option: Sprinkle remaining onions and cheese over unbaked casserole. Cover and freeze. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 375°. Bake casserole as directed, increasing time as necessary to heat through and for a thermometer inserted in center to read 165°.