Layer onion and garlic in the bottom of the slow cooker.
Heat oil in large skillet. Brown brats for about 1-2 minutes per side, just until browned. (They don’t need to be cooked through!) Nestle brats on top of the onions and garlic.
Combine beer, mustard and caraway seeds in a small bowl. Season with salt and pepper and pour over brats and veggies. Cook on high for 2 hours, or low for about 4 hours, or until brats are cooked through.