ED: Guessing here that you are supposed to mix the ground beef, chili sauce, mustard, horseradish, and onion.
Add salt and pepper to taste.
Chill at least 3 hours.
Butter hamburger buns, spread meat to edge.
ED: I think you are putting the bottom of the bun in a sheet pan, spreading it with the mix, and then putting on the top half of the bun. 
Bake in hot oven at 400° for 30 minutes.
ED: Not sure where we are going here. 40 minutes at 400° seems like it would burn the bread.