Slider burgers with mini meatballs

Ingredients:

• 900 g ground beef (preferably from the shoulder)
• 1/2 cup breadcrumbs
• 1/2 cup whole milk
• 2 finely chopped garlic cloves
• Coarse salt and freshly ground pepper
• 3 tbsp. extra virgin olive oil
• 1/2 onion, diced
• 680 g marinara sauce (see recipe) Here)
• 24 small round buns
• 2 tbsp. grated mozzarella cheese

Preparation:

1. In a large bowl, combine the ground meat, breadcrumbs, milk, garlic, 1/2 teaspoon salt, and black pepper to taste. Mix the ground meat with your hands. Form into meatballs, each about a full tablespoon in size.
2. In a large nonstick skillet, heat 2 tablespoons olive oil over medium heat.
   
   Add the onion and cook, stirring, for 2 minutes, until softened. Transfer to a plate. Heat the remaining 1 tablespoon of olive oil in a skillet over medium heat.
   
   Add the meatballs and cook, turning, until golden brown on all sides, about 1 minute. (If necessary, cook them in two batches.)
3. Pour in the marinara sauce and add the fried onions, stirring gently. Cover and simmer over low heat for 20 minutes.
4. Before serving, cut the buns and sprinkle the bottoms with mozzarella cheese. Top with meatballs, a little sauce, and more mozzarella cheese. Top with the top buns.