Melt butter in heavy saucepan. Add onion, rice, and mushrooms, sauté until rice starts to brown. Add stock, parsley, salt, and pepper; bring to boil. Reduce heat to low; cook, covered, 25 minutes or until all liquid is absorbed.
Grease 1-quart mixing bowl. Pack rice into bowl tightly. Keep warm while preparing tomatoes.
Split tomatoes in half.
Combine melted butter, bread crumbs, cheese, salt, pepper, and paprika; mix well. Sprinkle crumb mixture over tomatoes; broil in preheated broiler 4 inches from heat until golden.
Unmold rice onto warm serving dish; surround with tomatoes. Serve. Makes 6 servings.