Add both venison and sausage to a medium-sized mixing bowl. Sprinkle with gelatin, then combine the ingredients.
Add mustard, salt and pepper. Give the ingredients another quick mix.
Divide the meat into patties. Slightly indent the middle of each patty to keep it from swelling up while cooking.
Carefully add your venison burger patties to a hot cast iron skillet over medium heat. Cook the patty until a nice brown crust forms on the bottom, about 3 minutes. Flip, and cook an additional 4 or 5 minutes.
For the last two or so minutes of cooking, I like to add the cheddar cheese so it has time to melt.
Notes
If you don't have gelatin, you can substitute it for one egg.
Add a splash of Worcestershire sauce to the ground venison if you don't prefer the slightly gamey taste.
Use an instant thermometer to check the internal temperature to verify the meat is cooked through.