Optional toppings: Shredded lettuce, sliced tomato and red onion
3 tbsp. butter, softened
6 hamburger buns, split
2-1/2 tsp. hamburger seasoning, divided
1/3 cup beef broth
1-1/2 pounds ground beef
1/8 tsp. pepper
1/8 tsp. salt
2 tbsp. Dijon mustard
2 tbsp. prepared horseradish
1/4 cup mayonnaise
2 tbsp. butter