4 beef tenderloin steaks (1 inch thick and 4 ounces each)
1/2 teaspoon salt
1/4 teaspoon pepper
5 tablespoons butter, divided
1 large onion, halved and thinly sliced
1 cup beef stock
1 tablespoon Dijon mustard
Sprinkle steaks with salt and pepper. In a large skillet, heat 2 tablespoons butter over medium-high heat. Add steaks; cook 4-6 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Remove from pan; keep warm.
In same pan, heat 1 tablespoon butter over medium heat. Add onion; cook and stir 4-6 minutes or until tender. Stir in stock; bring to a boil. Cook 1-2 minutes or until liquid is reduced by half. Stir in mustard; remove from heat. Cube remaining butter; stir into sauce just until blended. Serve with steaks.