Slice your onion into half moons and smash your garlic cloves with the flat side of a knife. Layer both directly into the bottom of your slow cooker so they sit underneath the brats as everything cooks together.
Heat a little oil in a large skillet over medium-high heat and brown your brats for about one to two minutes per side. They don't need to cook through here, you're just building color and locking in flavor before the slow cooker takes over.
Once browned, transfer the brats directly on top of the onions and garlic already sitting in your slow cooker. This layering helps the brats soak up flavor from underneath as they continue cooking slowly.
In a small bowl, whisk together your beer, mustard, and caraway seeds, then season with salt and pepper. Pour this mixture evenly over the brats and vegetables so everything gets coated in that savory, slightly tangy liquid.
Cover your slow cooker and cook on high for about two hours, or on low for closer to four hours, until the brats are fully cooked through. Once done, they'll be tender, juicy, and ready to serve straight from the pot.