Preheat your oven to 400°F (200°C)

In a large bowl, combine 1/2 cup breadcrumbs and 1/4 cup milk, let it sit for about 5 minutes to soak

Add 1 lb ground beef, 1 egg, 1 clove minced garlic, 1/4 cup grated Parmesan cheese, 1 teaspoon dried oregano, 1/2 teaspoon salt, and 1/4 teaspoon black pepper to the breadcrumb mixture

Gently mix until just combined, being careful not to overwork the meat

Form into 12 meatballs, each about the size of a golf ball

Place meatballs on a baking sheet lined with parchment paper

Bake in the oven for 15-20 minutes or until fully cooked through (internal temperature should reach 160°F or 71°C)

While meatballs are cooking, split the 12 slider buns and lightly toast them if desired

Once meatballs are done, reduce oven temperature to 350°F (175°C) if necessary

Spoon a bit of 1 cup marinara sauce onto the bottom half of each bun, place a meatball on top, add another spoonful of sauce, then cover with a slice of mozzarella from 12 slices or as needed

Place the sliders back on the baking sheet or in a baking dish, close with the top half of the buns

Bake for about 5 minutes or until the cheese is melted and bubbly