In a large skillet over medium-high heat, brown the ground beef and ground sausage. When the meat is halfway browned, add the butter, mushrooms, and onion. Cook until the meat and mushrooms are browned and the onions are tender.
Reduce the heat to medium-low. Add the garlic and marjoram, cooking for 1 minute while stirring. Mix in the marinara and season with kosher salt and fresh ground black pepper. Let it simmer for about 7-10 minutes.
Meanwhile, split and toast the English muffins on a baking sheet lined with aluminum foil under the broiler. Evenly distribute the meat mixture over the toasted muffins. Top with diced pepperoni and sprinkle with mozzarella cheese. Broil for about 2-3 minutes or until the cheese is melted.