Mix ground beef, onion, salt, and pepper to make the patty mixture.
Shape the mixture into small round mini patties.
Heat cooking oil in a pan and cook the patties on both sides.
Wash cherry tomatoes and baby greens thoroughly.
Cut the mini buns in half.
Assemble the burger by placing a patty, cheese, baby greens, and cherry tomato on the bun, and cover with the top bun.
Secure with a skewer.