Heat large saute pan over medium-high heat. Add the ground beef and cook until browned, breaking up the meat with a spoon into smaller chunks. Season with salt and pepper. Remove the beef, draining all but 1 tablespoon of the fat from the pan.
To the same pan, add the onion and garlic and cook until softened, about 5 minutes. Add the crushed tomatoes – balsamic vinegar and bring to a simmer. Add the ground beef back to the pan and reduce heat to low. Continue to cook for 30 minutes or until the sauce has reduced. Season with salt and pepper.
Meanwhile, prepare the spinach. Heat 1 tablespoon oil in a saute pan over medium heat. Add the spinach and garlic and saute until the spinach has wilted, about 2-3 minutes. Season with salt and pepper. Set aside.
Preheat broiler to high.
Assemble the sliders. Top the bottom bun with spinach and a spoonful of the sloppy joe filling. Place a slice of fresh mozzarella on top and broil until cheese has melted, about 1-2 minutes. Top sliders with the remaining bun and serve.