Mix catsup, brown sugar, lemon juice, Worcestershire sauce, mustard, onion salt and pepper in 2 cup glass measure.
Microwave 1 1/2 minutes; stir.
Microwave to boiling, 1 to 2 minutes.
Layer half of the beef slices and half of the sauce in 1 quart casserole; repeat.
Cover and microwave until hot and bubbly, 1 1/2 to 2 1/2 minutes.
Place bottom halves of buns on paper serving plate.
Top with beef and remaining bun halves.
Microwave uncovered until buns are hot, 30 seconds to 1 minute.