Cook beef in a large nonstick skillet over medium-high heat until browned; stir to crumble. Drain well; return meat to pan.
Add 3/4 cup chopped onion and broth to pan; bring to a boil. Reduce heat, and simmer 25 minutes or until liquid is absorbed. Remove from heat; stir in pepper.
Spread mustard evenly over bottom and top half of each bun. Arrange pickle slices on bottom halves of buns; top evenly with 1/2 cup onion. Spoon 1/2 cup meat mixture onto bottom half of each bun; top with remaining halves.