Cook the bacon in a large skillet over medium heat until cooked (but not crispy). Transfer to a paper towel-lined plate to drain (reserve pan and bacon grease). Once cool enough to handle, chop into small pieces.
Combine the bacon, onion, mushrooms, salt, pepper and beef in a large bowl and mix well.
Gently shape mixture into patties about 3/4" thick. Using your thumb, make a shallow depression in the center of each burger to prevent puffing up during cooking.
Re-heat the skillet with the bacon grease over med-high heat. When oil is hot again, add the burger patties and cook 3-4 minutes per side (or until desired doneness).
Serve in buns with desired toppings.
Nutrition Info Per Serving: Calories 468 Total Fat 24 g Cholesterol 100 mg Sodium 693 mg Potassium 551 mg Total Carbohydrate 22 g Dietary Fiber 1 g Sugars 3 g Protein 38 g