1. Prepare the Tomato Beef Sauce

• Preheat your oven to 400℉ (200℃).
• Heat olive oil in a large skillet over low heat. Sauté the chopped onion for 5-7 minutes until soft and translucent.
• Add the ground beef and cook, stirring constantly, until no pink remains.
• Stir in oregano, salt, tomato passata, and basil pesto. Bring to a gentle boil, then reduce heat and simmer for 20 minutes, stirring occasionally. Remove from heat and allow the sauce to cool slightly.

2. Cook the Pasta Shells

• Cook the pasta shells in a large pot of salted boiling water, following package instructions. To ensure the shells don’t overcook, reduce the cooking time by 1-2 minutes.
• Drain the shells and rinse them with cold water to prevent sticking. Let them cool to room temperature.

3. Assemble the Dish

• Spread 1 cup of the prepared sauce on the bottom of a 9×13-inch baking dish to create a base layer.
• Arrange the cooked shells in a single layer, open side up. Spoon approximately 2 teaspoons of the sauce mixture into each shell.
• If you have extra sauce, spread it over the shells. Sprinkle the grated mozzarella evenly on top.

4. Bake

• Cover the dish with foil and bake in the preheated oven for 10 minutes.
• Remove the foil and bake for an additional 15 minutes until the cheese is melted, golden, and bubbly.