In a medium saucepan, melt 1 tablespoon butter or margarine. Add 1/4 cup chopped onion, and saute until the onion is clear and beginning to brown. Add 10.75 oz. can of cream of celery soup and 1/2 cup of reserved tomato juice. Stir until smooth. Add 1 cup shredded sharp Cheddar cheese and a 14.5 oz can of drained, diced tomatoes. Cook over low heat, stirring occasionally, until the cheese melts. Combine the 10 oz. of prepared tortellini with the cheese sauce mixture, and mix well. Pour into a 1 1/2-quart casserole dish and bake at 350 degrees until hot and bubbly. Serve immediately as a side dish, or as your entree! It is hearty and filling! ...And delicious!!!