Keep in mind that what's important here is the technique and the grind, not so much my additional ingredients. Of course, I love my venison burgers like this, so I am biased. But so long as you follow general guidelines on toppings: mix something rich (cheese) with something sharp (tomato) and something slightly bitter or cleansing (lettuce or sorrel leaves) and a touch of sweet (ketchup) and you will be in good shape.
Condiment of your choice (ketchupmustard, remoulade, mayo, etc)
Make sure the meat and bacon are cold. Cut the venison into chunks that will fit into your grinder. Do the same for the bacon. Mix the two together roughly so you can add a bit of each into the grinder as you go. Grind 1/2 to 2/3 of the mixture coarsely and the rest with the fine die. NOTE: If you are grilling your venison burgers, flip this so you grind 2/3 of the mix fine and only 1/3 coarse -- the reason is because grilled burgers tend to cook better and stay juicier when the grind is fine.
Make between 4 and 6 patties, depending on how large you want your burgers. Form the patties with only as much force as absolutely needed -- you want the patties to hold together only loosely. Make them about 1/2 to 1 inch thick. Use your thumb to press an indentation into the center of each patty: This prevents the burgers from turning spherical when you cook them. Set the burgers aside.
Heat the butter in a frying pan over medium-high heat. When it's hot, add the sliced onion and cook until it's done to your liking. Some people like juicy onion with a little char on the edges, some people prefer to go the full caramelized onion route. When finished, put the onions in a bowl so you can have them ready.
I prefer grilled burgers, so I'll go through that method. Heat your grill on high and be sure to scrape down the grates with a wire brush. Only salt your burgers right before you cook them, and if you are salt-sensitive you might not need to with these because of the bacon. Place the patties on the grill and cook them without disturbing them (with the grill cover open) for 3 to 5 minutes, depending on how well done you like your burgers. Flip and cook for the same amount on the other side. I prefer 3 minutes per side with a really hot grill.
When you flip the burgers, grind some black pepper over them, then spoon a little caramelized onion on each one if you'd like. With about 90 seconds to go on the second side, lay the cheese on top of the onions and cover the grill until the burger's ready. If you like toasted buns, toast them on the grill in this last 90 seconds. When everything's done, move the burgers and buns to a sheet tray or plate so the meat can rest for 5 minutes, while you build the burgers.
You can do this any way you want, but I start with a green thing, then some ketchup or mayo or whatever, then the burger patty that has the onions and cheese already on it, topped with a slice of tomato (or roasted red peppeand finally some more of whatever condiment I happen to be using. My method is just how I do things; you can do anything you'd like.