Prepare the beef mixture:
In a skillet, brown the ground beef over medium heat until fully cooked, breaking it up into crumbles as it cooks. Drain any excess fat. Add the chopped onion and minced garlic to the pan and sauté until softened, about 3-4 minutes.

Slow cook the beef:
Transfer the cooked beef mixture to a slow cooker. Add the pizza sauce, oregano, basil, red pepper flakes (if using), and salt and pepper to taste. Stir everything together and cover. Cook on low for 3-4 hours or on high for 1 1/2 – 2 hours, until the flavors meld together.

Assemble the burgers:
When ready to serve, preheat your oven broiler. Place the hamburger buns on a baking sheet and toast them under the broiler for 1-2 minutes until lightly browned.

Top the burgers:
Spoon the slow-cooked beef mixture onto the toasted buns. Sprinkle each burger with shredded mozzarella and grated Parmesan cheese. Place the burgers under the broiler for an additional 2-3 minutes, until the cheese is melted and bubbly.

Serve:
Garnish with fresh basil leaves (optional) and serve with your favorite side dishes. Enjoy your cheesy, pizza-inspired burgers!