In a medium bowl, mix together the ground beef, onion, bread crumbs, egg, and milk using your hands. Shape into 4 patties.
Heat the oil in a large skillet over medium-high heat. Dredge the patties in flour just to coat, and quickly brown on both sides in the hot skillet. Place browned patties into the slow cooker stacking alternately like a pyramid. In a medium bowl, mix together the sour cream and boullion. Pour over the meat. Cook on the Low setting for 4 or 5 hours, until ground beef is well done.