Set your oven to broil (with the oven rack in the upper-middle position), and line a sheet pan with foil for easy clean-up.
In a large bowl, combine beef, salt, pepper, mustard and egg.
Form patties and place them on a baking sheet.
Place in oven under a high broiler for 5 minutes.
Cover with foil. Keep them warm in a 250 degree oven for up to 30 minutes.
This recipe is for a slider-sized burger. Ingredients for full-sized burgers are the same, simply bake under the broiler for four minutes, flip them and place back in the oven under broiler for 3-4 more minutes. Replace suggested cocktail buns with full-size buns and you're good to go!
Don't overwork your meat mixture.
Line your baking sheet for easy clean-up.
Keep them warm in the oven, covered in foil for up to 30 minutes at 250 degrees.