Preheat a grill pan or seasoned cast iron skillet over medium high heat. Brush the pan with cooking oil.
Grill the bratwurst sausages for about 3 to 4 minutes on each side, or just until the sausages are nicely browned. Remove the brats from the grill and set them aside.
Place the sauerkraut (optional), sliced apple (optional), sliced onions and minced garlic in the bottom of the slow cooker.
Add the chopped parsley, celery salt, light brown sugar, caraway seeds (optional), and black pepper over the onions and garlic.
Heat the butter in a small saucepan until it melts. Stir in the Dijon mustard or coarse ground mustard and honey until well combined. Remove the saucepan from heat.
Place the grilled bratwurst sausages on top of the sauerkraut (optional), onions and seasonings in the slow cooker.
Pour the beer (or chicken broth or apple cider) over the sausages, allowing it to cover most of the brats. Drizzle the honey mustard mixture and apple cider (optional) over the sausages.
Cover the slow cooker with a lid and cook on low for 4 to 5 hours or on high for 2 to 3 hours. This will allow the brats to cook through and absorb all the flavors. The bratwurst should be cooked to an internal temperature of 160°F, as measured by an instant read thermometer.
Serve slow cooker bratwurst immediately with your favorite condiments, buns, side dishes, or alongside other game day snacks.