Directions

1. Preheat the oven to 350 degrees F (175 degrees C).
2. Bring a large pot of lightly salted water to a boil. Stir in shells and return to a boil. Cook pasta uncovered, stirring occasionally, until tender yet firm to the bite, about 10 minutes. Drain shells and set aside.
3. Heat a large skillet over medium-high heat. Cook and stir ground beef in the hot skillet until browned and crumbly, 5 to 7 minutes. Add mushrooms; cook and stir until mushrooms have softened and released their liquid, about 5 minutes. Drain and discard excess fat. Transfer meat mixture to a separate bowl and allow to cool to room temperature, about 10 minutes.
4. Stir ricotta cheese, egg, garlic, and oregano into cooled meat mixture until well combined. Season with salt and pepper.
5. Add 1/2 of tomato sauce to the bottom of a 9x13 baking dish. Fill each shell with about 1 teaspoon meat mixture and place filling-side-up in the baking dish. Repeat with remaining shells and filling. Spread remaining tomato sauce over shells, and sprinkle with shredded mozzarella cheese.
6. Bake in the preheated oven until cheese is melted and starting to brown, approximately 25 minutes. Serve immediately.