Place rack in center of oven then preheat to 400 degrees. Meanwhile, lightly coat baking sheet with non-stick spray then set aside. Place ground beef into a large mixing bowl, add onion then set aside.
In a small bowl, whisk together milk, eggs, bouillon, pepper and Worcestershire sauce until the eggs are well beaten and the bouillon has dissolved. Pour the liquid mixture over the beef then mix until combined.
Add breadcrumbs into the beef mixture then mix until they are evenly distributed and the mixture is thick.
Roll the meat into 24 golf ball-sized meatballs, place onto prepared baking sheet then bake 18–20 minutes until nicely browned. For uniform balls, you can use a medium cookie scoop.
While the meatballs bake, bring marinara sauce to a simmer in small saucepan. Cut each slice of cheese into four squares. Set aside. Using two 9" x13" baking pans, place bottom half of 12 slider buns in each pan. Spoon marinara on each bun then place a meatball on top. Spoon more marinara on top of each meatball and add a cheese square on each meatball. Add tops to each sandwich, cover pans with foil, and bake 10 minutes. Remove foil and bake 10 minutes more until cheese is completely melted.
Stick a basil leaf under each meatball for garnish, place on a serving platter and enjoy.
One Last Thing
If you’re short on time, purchase pre-made meatballs from the supermarket then just heat and assemble!