"This massive venison burger is sure to do you in. Pile it high with a fried egg, maple bacon, avocado, caramelized onions, sauteed 'shrooms, thick slab of cheese -- and rest it on a foundation of iceberg lettuce with fresh tomato. Don't forget the toasted Kaiser roll and onion rings. Welcome to the World of Gluttony. Fire Dept. tested and approved."
In a bowl, thoroughly mix together the ground venison, pork sausage, 2 tablespoons barbecue sauce, salt, and black pepper. Divide the mixture into fourths, and form each fourth into a large patty. Refrigerate until time to grill.
If using onion rings, preheat oven to 425 degrees F (220 degrees C). Spread frozen rings in a single layer on a baking sheet, and bake until hot and golden brown, about 15 minutes. Set the onion rings aside.
In a large skillet, heat butter and olive oil; cook the onions until they begin to brown, stirring occasionally, about 10 minutes; sprinkle with salt and pepper. Mix in the sliced mushrooms, and cook, stirring occasionally, until browned, 10 to 15 more minutes. Set the onions and mushrooms aside.
Place the bacon in a large, deep skillet, and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on a paper towel-lined plate.
Heat 1 tablespoon of butter in a clean nonstick skillet over medium heat until the foam disappears. Crack the eggs into a bowl, and gently pour them into the butter. Pan-fry the eggs until the whites are opaque and the eggs slide around the pan easily, about 1 minute; carefully flip each egg over in the skillet, avoiding breaking the yolk, and pan-fry for about 1 more minute. The whites should be firm but the yolks should be runny. Transfer the eggs to a plate.
Preheat an outdoor grill for high heat, and lightly oil the grate. Grill the venison burgers until they show grill marks and are cooked to your desired degree of doneness, 5 to 8 minutes per side for well-done. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C). While still on the grill, gently top each burger with a fried egg and a slice of cheese. Lower the grill lid, and cook until the cheese has melted, about 1 minute.
To assemble, place the bottom half of a Kaiser roll onto a plate; spread with a layer of shredded lettuce, followed by tomato slices, the venison burger, a layer of mushrooms and onions, bacon slices, onion rings, avocado slices, ketchup, mustard, and mayonnaise. Place the roll top onto the sandwich. Good luck.