1. Preheat grill to high heat.
2. Combine first 5 ingredients in a bowl. Divide buffalo mixture into 4 equal portions, gently shaping each into a 1/2-inch-thick patty. Press a nickel-sized indentation in the center of each patty.
3. Place patties on grill rack coated with cooking spray; grill 3 minutes on each side or until desired degree of doneness.
4. Place bottom bun halves on plates. Top each with 1/4 cup arugula, 1 onion slice, and 1 patty. Spread 1 tablespoon ketchup on top half of each bun; place on top of burgers.