In a large skillet over medium heat, combine the ground beef and Italian sausage. Cook until browned, breaking up any clumps with a wooden spoon, about 8-10 minutes.
Add the chopped onion and minced garlic to the meat mixture, sautéing until the onion is softened, about 3-4 minutes.
Stir in the dried oregano, Italian seasoning, and red pepper flakes (if using). Cook for an additional minute to allow the spices to bloom.
Pour the marinara sauce into the skillet, stirring to combine. Let the mixture simmer for about 5 minutes, allowing flavors to meld.
Preheat your oven to 375°F (190°C).
Slice the slider buns in half and arrange the bottom halves in a baking dish. Spoon the meat mixture generously over the bottom buns, spreading it evenly.
Sprinkle the shredded mozzarella cheese and grated Parmesan cheese on top of the meat mixture.
Place the top halves of the buns over the cheese layer and press down gently.
Cover the dish with aluminum foil and bake for 15 minutes. Then, remove the foil and bake for an additional 10 minutes or until the tops are golden brown and the cheese is bubbly.
Remove from the oven and let cool for a few minutes. Garnish with fresh basil leaves before serving.
Notes
Feel free to adjust the heat level by adding more or less red pepper flakes.