Decide if you are going to make your Cowboy Beans in a slow cooker or bake in the oven in a large dish or cast iron dutch oven.
Chop the sliced uncooked bacon into small pieces, over medium heat, brown the bacon in a heavy cast iron skillet or a large cast iron dutch oven.
Remove the crispy cooked bacon to a small bowl, don't use the bacon grease in this recipe. Next add the chopped onion and brown for 5-6 minutes until translucent, add the minced garlic, stir for one minute. Remove the onion and garlic in a small bowl with the crispy cooked bacon.
Next, brown the ground beef, cook for 6-8 minutes. Next, add the bacon, onions, and garlic back with the ground beef. Place into the slow cooker (or large dutch oven or large dish if baking in oven).
Next, add the beans; 3 cans baked beans (don't drain), 2 cans kidney beans (drain), 1 can cannellini beans (drain), and 2 cans black beans (drain).
Add brown sugar, cider vinegar, ketchup, and yellow mustard, stir well to combine.
Cook in slow/cooker crockpot on low for 2 hours or bake 350 degrees for 1 ½ hours in a large covered dish. Serve hot.