In small skillet, saute onion in hot oil until onion is tender. Inlg. bowl, combine the onion and tuna, bread crumbs, 1/2 cup milk, lemon juice, salt, and pepper. Mix well. In another lg. bowl, combine cooked macaroni with soup, 2/3 cup milk, mustard, and worcestershire sauce. Mix well. Spread half of macaroni mixture over bottom of lightly greased 12"x8" baking dish. Spread tuna mixture over macaroni layer; top evenly with remaining macaroni mixture. Sprinkle with cheese. Bake in 350~ oven for 30 mins. or until casserole is bubbly and cheese topping is melted and lightly browned. Makes 4 to 6 servings.