Heat the oven to 375 degrees. Spray 12 regular-size muffin cups with non-stick cooking spray. In a 10-inch skillet, cook and stir the ground beef and onion over medium-high heat for 8 minutes or until the beef is brown; drain. In a medium bowl, stir together the beef mixture, ketchup, mustard and salt. Set aside. Unroll the dough (if using crescent rolls, firmly press perforations to seal). Press into a 12x8-inch rectangle. Cut into 4 rows by 3 rows to form 12 pieces. Press each piece gently into the muffin cups. Spoon 3 tablespoons of the beef mixture into the dough cups. Bake for 12 to 17 minutes or until the cups are golden. Sprinkle the top of each the cheese. Bake for 1 to 2 minutes longer or until the cheese begins to melt. Top each with diced tomato, a dollop of sour cream and a few shreds of lettuce.