Almost-Instant Pierogi
["3 lb. potatoes, peeled and quartered", "8 oz. sharp Cheddar or Colby cheese, shredded", "1 tsp. salt", "1/2 tsp. pepper", "1/2 c. butter", "6 c. sliced onions", "1 (12 oz.) box jumbo stuffing shells, cooked and drained"]
["Cook potatoes in boiling water until tender.", "Drain and return to pot.", "Add cheese to hot potatoes and mash until potatoes are smooth.", "Add salt and pepper.", "Melt butter in a large skillet. Add onions and cook 15 minutes until tender.", "Heat oven to 350\u00b0. Have 2 (13 x 9-inch) baking dishes ready.", "Spread half the onions over the bottom of baking dishes.", "Stuff shells and arrange over onions.", "Spread remaining onions over top.", "Bake uncovered 15 to 20 minutes until heated through."]
["potatoes", "Cheddar", "salt", "pepper", "butter", "onions", "jumbo stuffing shells"]