To prepare aioli, combine first 5 ingredients in a food processor; pulse 1 minute or until smooth. Cover and chill.
Prepare grill or broiler.
To prepare the burgers, place onion slices on a grill rack or broiler pan coated with cooking spray, and cook 2 minutes on each side. Set aside.
Combine the beef and the next 9 ingredients (beef through crushed garlic) in a large bowl. Divide the beef mixture into 5 equal portions, shaping each portion into a 1/2-inch-thick patty. Place patties on grill rack or broiler pan coated with cooking spray, and cook for 6 minutes on each side or until burgers are done. Spread 1 1/2 tablespoons aioli over top half of each bun. Place patties on bottom halves of buns, and top each with 1 onion slice and top half of bun.