Heat a lightly oiled cast iron skillet over medium heat until hot. Add the bratwurst and sear until browned on each side. Remove from the skillet.
Add the sauerkraut, potatoes, and onions to the slow cooker. Add the bay leaves, garlic, mustard seed, and caraway. Nestle the bratwurst into the sauerkraut, then cover with beer.
Cover and cook for 8 hours on low, until the potatoes are tender and the bratwursts are cooked through.