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.