It'S All In The Burger

• 1 teaspoon butter
• 1 cup finely chopped red onion
• 3 pounds ground beef
• 1 cup chopped mushrooms
• 1/2 cup ketchup
• 4 egg yolks
• 1/4 cup Worcestershire sauce
• 2 tablespoons horseradish mustard
• 4 teaspoons ground black pepper
• 1 tablespoon sea salt

1. Heat butter in a skillet over medium heat and cook and stir red onion until soft and translucent, about 5 minutes. Mix ground beef, cooked onion, mushrooms, ketchup, egg yolks, Worcestershire sauce, horseradish mustard, black pepper, and sea salt in a large bowl until thoroughly combined.
2. Form the mixture into six 3/4-inch thick patties and cook in a large skillet over medium heat until burgers are juicy and browned, 5 to 7 minutes per side. An instant-read meat thermometer inserted into the center of a patty should read at least 160 degrees F (70 degrees C).

butter, red onion, ground beef, mushrooms, ketchup, egg yolks, worcestershire sauce, horseradish mustard, ground black pepper, salt

Taken from www.allrecipes.com/recipe/222779/its-all-in-the-burger/ (may not work)