1. Place the sliced onions and apples at the bottom of the slow cooker.
2. Lay the bratwurst sausages on top of the onion and apple slices.
3. Sprinkle mustard seeds, salt, and pepper over the sausages.
4. Pour the beer and apple cider vinegar over everything.
5. Set the slow cooker to low and cook for 6-7 hours or on high for 3-4 hours, until the brats are fully cooked and the flavors meld together beautifully.
6. Serve the brats on buns if desired, topped with the onions and apples from the cooker, and your choice of condiments.
