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1
Fill a large pot with lightly salted water and bring to a rolling boil. Stir in shells and return to a boil. Cook pasta uncovered, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Drain carefully.
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2
Heat butter and olive oil in a large skillet over medium-low heat. Add garlic; cook and stir until lightly browned, about 5 minutes. Add mushrooms and vinegar. Cook, stirring occasionally, until mushrooms are soft, about 5 minutes more. Drain any excess liquid and transfer mushrooms and garlic to a large mixing bowl to cool.
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3
Rinse and dry skillet. Add olive oil over medium heat. Add garlic and cook until lightly browned, about 2 minutes. Add spinach and cook until wilted, 3 to 4 minutes. Add spinach to mushroom mixture in the mixing bowl.
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4
Add ricotta, Italian cheese blend, Parmesan cheese, egg, basil, salt, and pepper to the spinach and mushroom mixture. Mix thoroughly to combine.
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5
Preheat the oven to 375 degrees F (190 degrees C).
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6
Open canned tomatoes and add basil, salt, oregano, and red pepper flakes directly to the can. Pour 1/4 of the sauce into a 9x13-inch baking pan.
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7
Stuff each pasta shell with a large spoonful of filling and place, filling-side up, in the baking pan. Cover with remaining sauce and tightly cover with aluminum foil.
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8
Bake in the preheated oven until heated through, about 30 minutes.
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9
Sprinkle 2 tablespoons Parmesan cheese on top. Return pan to the oven, increase temperature to 400 degrees F (200 degrees C), and cook until bubbling, 15 to 20 minutes more.
