1. Begin by preheating your slow cooker on the low setting.
2. In a skillet, melt the butter over medium heat. Add the sliced onions and garlic, and sauté until they are soft and fragrant, about 5 minutes.
3. Place the bratwursts in the bottom of the slow cooker. Add the sautéed onions and garlic over the top.
4. Pour the beer over the brats and onions.
5. Add the sauerkraut, caraway seeds, black pepper, salt, and brown sugar to the slow cooker, spreading them evenly.
6. Cover the slow cooker and cook on low for 6-7 hours, or until the brats are cooked through and flavors have melded together beautifully.
7. Serve the beer brats hot, accompanied by the sauerkraut and onions.