PREHEAT oven to 350°F Cook beef in large skillet until brown; drain. Add seasoning mix and water; cook over low heat for 5 minutes or until thickened. Add beans and 3/4-cup cheese; mix well. Spread 1/4-cup salsa over bottom of each of two 8-inch-square (2-quart) baking dishes.
FILL pasta shells with beef mixture; place half of shells in each baking dish, filled side up. Spoon remaining salsa over shells. Cover one baking dish with plastic wrap, then wrap with foil. Label and freeze for up to 2 months.
COVER remaining baking dish with foil. Bake for 40 to 45 minutes or until heated through. Sprinkle with green onions and remaining cheese. Serve with sour cream.
TO BAKE FROZEN SHELLS: Place baking dish in the refrigerator overnight. Preheat oven to 375°F Remove plastic wrap; recover with foil. Bake for 45 to 50 minutes or until heated through. Sprinkle with 1/4 cup sliced green onions and 1/4 cup shredded reduced-fat cheddar cheese. Serve with sour cream.