Decide if you are going to make them in a slow cooker or bake in the oven. You can use a deep casserole dish or large pot like a dutch oven. I like a large heavy cast iron pot so I can cook in it and finish baking all in one. Slow cookers are great for parties to keep them warm for several hours.
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 on the stove top using a wooden spoon. Tip, you can cut the bacon into small pieces with clean kitchen scissors right into the pot-I find this faster!
Remove the crispy cooked bacon to a small bowl with a slotted spoon. Don't remove the bacon drippings from the pan. Next add the chopped onion and brown for 5-6 minutes until translucent, add the minced garlic, stir for one minute.
Next, brown the ground beef, cook for 6-8 minutes. Next, add the bacon, back in with the cooked ground beef, onion, and garlic.
Next, add 3 cans of baked beans (don't drain), 2 cans of kidney beans (drain), 1 can of cannellini beans (drain), and 2 cans of black beans (drain).
Add the remaining ingredients; brown sugar, cider vinegar, ketchup, and yellow mustard. Stir all the ingredients well to combine.
Place into the slow cooker or large dutch oven or large deep casserole dish. The baked beans usually doesn't all fit in one standard 9 x 13, you need a deeper dish.
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.