Arrange frozen biscuits, with sides touching, in 3 rows of 4 biscuits on a lightly floured surface. Let stand 30 to 45 minutes or until biscuits are thawed but cool to the touch.
Sprinkle thawed biscuits lightly with flour. Press biscuit edges together, and pat to form a 10- x 12-inch rectangle of dough. Stir together softened butter, mustard, and onion. Spread butter mixture evenly over thawed dough; sprinkle evenly with Swiss cheese and ham.
Roll up, starting at one long end; cut into 12 (about 1-inch-thick) slices. Place one slice into each of 12 lightly greased 3-inch muffin pan cups.
Bake at 375° for 20 to 25 minutes or until golden brown. Cool slightly, and remove from pan.