In a sauté pan over medium heat, add olive oil. When hot, add onions and cook until they begin to lightly brown, about 2-3 minutes.
Crumble ground beef into pan and cook until browned, about 5 minutes. Optionally, drain any excess grease from the pan at this time.
Add marinara sauce, tomato paste, worcestershire sauce, Italian seasoning, salt, and red pepper flakes. Stir to combine. Cook for another 2-3 minutes, or until ingredients are heated through and sauce has thickened to your liking. Top with cheese slices and remove from heat.
Once cheese has melted, transfer sloppy joe mixture onto toasted buns and serve.
Marinara sauce can be substituted with spaghetti sauce. Traditional marinara sauce includes garlic. If your sauce is not particularly flavorful, add 2 cloves of minced garlic with your ground beef in the last 30 seconds of cooking (step 2, before adding your sauce).
Leftover sloppy joes will keep for 3-4 days in a tightly sealed container in the refrigerator.
Provolone can be substituted with parmesan, mozzarella, or a combination of multiple cheeses.