In a large bowl, add the ground beef, salt, mustard, garlic, paprika, pepper, and ketchup. Mix well.
Split the mixture into 12 balls and press into 12 holes of a muffin pan. Press the mixture evenly along the bottom and up the side to form a cup shape. I recommend wearing food-safe disposable gloves when doing this.
Place a small amount of cheese in the base of each cup then layer in the mushrooms, bacon (reserve a small amount for garnish, if desired), onion, and pickle slices. Top with the rest of the cheddar.
Bake for 15-20 minutes until the cups have shrunk away from the sides and the cheese is bubbling.
Using a spatula or a fork, gently lift each of the cups from the muffin pan. There will be some liquid left, so take care not to burn yourself.
Place your cups on a serving tray, drizzle with the burger sauce and top with reserved bacon and scallions, if desired.