1. Peel and finely chop the onions. Brown them in a pan with 1/2 tablespoon of extra virgin olive oil.

2. Cut the cheese into triangles.

3. Heat the grill in the oven. Place the mini hamburger buns on the oven tray and toast under the grill.

4. Mix the ground beef, cooked onions, and incorporate the egg yolks. Season with salt and pepper.

5. Shape 12 mini meatballs. Flatten them slightly and cook in a little extra virgin olive oil for about 3 minutes on each side. Drain on paper towels.

6. Spread the base of the buns with ketchup, add slices of tomato, beef meatballs, and cheese.

7. Finish with the top of the buns. Secure with toothpicks and serve.