Spray a large skillet with cooking spray and place over medium-high heat. Add diced onion and minced garlic, and sauté until onion is translucent.

Add ground beef to the skillet and brown thoroughly, breaking it up with a wooden spoon. Drain the beef mixture, then return to skillet. Add refried beans, melted Velveeta cheese, cumin, tomato paste, and drained green chilies. Stir until combined, adding milk as needed to achieve a spreadable consistency.

Spread sour cream on each hamburger bun. Spoon the beef mixture onto the buns and garnish with shredded lettuce, diced tomatoes, and sliced black olives. Serve immediately.