Place sliced onions and garlic at the bottom of the slow cooker.
Season brisket with salt, pepper, and smoked paprika. Lay it over the onions.
In a bowl, whisk together BBQ sauce, Worcestershire sauce, and brown sugar. Pour over the brisket.
Cover and cook on low for 8 to 10 hours or until the brisket is fork-tender.
Remove brisket, slice against the grain, and spoon sauce and onions over the top before serving.