In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the 1/4 cup of finely diced onion and sauté until translucent, about 3-4 minutes. Stir in the 1 clove of grated garlic and sauté for another minute, until fragrant.
In a mixing bowl, combine the sautéed onion and garlic with 1/2 pound of ground beef, and season with salt and pepper to taste. Mix until just combined, being careful not to overwork the meat. Form into meatballs, about 1-1.5 inches in diameter.
Add the meatballs to the skillet and cook until browned on all sides, about 5-7 minutes. You may need to do this in batches depending on the size of your skillet.
Once the meatballs are browned, pour in the 1 1/2 cups of pizza sauce and add 1/2 cup of water if your sauce is too thick. Bring to a simmer and let cook for about 10 minutes, allowing the flavors to meld.
Preheat your oven to 350°F (175°C). Take your rolls and place them on a baking sheet. Spoon the meatballs and sauce into each roll, then top with 1 cup of grated mozzarella cheese and 16 slices of pepperoni.
Bake the assembled sandwiches in the preheated oven for about 10-15 minutes, or until the cheese is melted and bubbly. You can broil them for an additional 2-3 minutes for a crispy top.
Remove the sandwiches from the oven and let them cool for a minute before serving. Enjoy your Pepperoni Pizza Meatball Sandwich with your favorite sides or on its own!