Heat up a skillet with some fat. I used duck fat. If you don't have this, you need to get it. It's insanely good.
While your skillet heats up, dice up your onions and add them to the skillet. Let them cook for a bit, and while they do, dice your mushrooms and sundried tomatoes and add them to the skillet with the onions. Mix together and add apple cider vinegar to the skillet. Let cook down for a bit.
While those ingredients are mixing together, pull out the ground beef and get ready to get your hands dirty. Make lacrosse ball sized balls of meat and then flatten them out with your hand. You will need an equal number. My amount of meat (I had about 1.33lbs) made 4 burgers, so 8 flattened patties.
Add a bit of salt and pepper to taste to your mushroom, onion, and sundried tomato mixture. Once the onions are translucent and the mushrooms have cooked down, add a spoonful of the mixture to a patty and top another patty over that patty. Close the patties up by pinching the sides until the patties are tightly sealed. Sprinkle the tops of the patties with garlic and onion powder.
Once the burgers are sealed up, heat up a large skillet with 1-2 tablespoons of fat. Add burgers to pan and let cook on both sides for about 4-6 minutes. I covered the burgers with a lid to help them cook a little quicker AND, I swear they stay a bit more moist and tender that way.
Recipe by PaleOMG - Paleo Recipes at http://paleomg.com/over-stuffed-burgers/