Heat oven to 350°F. Grease cookie sheet. In medium bowl, combine tuna, onion, mayonnaise, salt and pepper; mix well.
Separate dough into 8 biscuits. Place 4 biscuits on greased cookie sheet. Press or roll each to form 5-inch round. Spoon tuna mixture in center of biscuits. Top each with cheese. Press or roll remaining 4 biscuits to form 5-inch rounds. Place over filling. Press edges to seal.
Bake at 350°F. for 15 to 20 minutes or until golden brown. Cut each sandwich in half. Top each with sour cream, tomato and lettuce.