Preheat your oven to 375°F and cook the jumbo pasta shells according to package directions until just barely al dente. Drain the shells carefully and lay them flat on parchment paper to cool slightly so they don't tear during stuffing.

Heat olive oil in a large skillet over medium-high heat and add the diced onion. Sauté for 3 minutes until softened, then add minced garlic and cook for another 1-2 minutes until fragrant.

Add ground beef to the skillet and break it into small pieces while cooking for 5-7 minutes until no pink remains. Drain excess fat from the pan before proceeding with the filling.

In a large mixing bowl, combine ricotta cheese, 1.5 cups mozzarella, parmesan, egg, dried basil, oregano, salt, and black pepper. Stir until everything blends smoothly, then fold in the cooked beef mixture until the cozy stuffed shells warm recipe filling distributes evenly.

Spread a thin layer of marinara sauce across the bottom of a 9x13-inch baking dish. Carefully spoon about 2-3 tablespoons of the filling into each pasta shell and arrange them seam-side up in the baking dish.

Pour the remaining marinara sauce evenly over the shells, then pour the beef broth around the edges to keep everything moist during baking. Sprinkle the remaining 1/2 cup mozzarella over the top.

Cover the baking dish with foil and bake for 30 minutes covered, then remove the foil and bake for another 15 minutes until the cheese on top melts and bubbles at the edges.

Remove from the oven and let rest for 5 minutes before serving to allow the shells to set slightly.