Preheat to 375°F and lightly grease a 9x13-inch baking dish with olive oil.

Warm olive oil in a skillet over medium-high heat, then cook minced garlic and sliced mushrooms until golden brown, about 5 minutes.

Add baby spinach to the skillet, tossing until wilted, about 2 minutes. Transfer mixture to a bowl and let cool slightly.

In a large bowl, combine ricotta, Parmesan, egg, and cooled veggies. Stir until smooth, then season with salt and pepper.

Boil jumbo pasta shells in salted water until al dente, about 8-10 minutes. Drain carefully and arrange open-side up in the baking dish.

Fill each shell with the cheese and veggie mixture, pressing gently to fit as much filling as possible.

Spread half of the marinara sauce in the bottom of the dish, top shells with remaining sauce, then sprinkle extra mozzarella over the shells.

Bake for 20-25 minutes, until cheese is bubbly and slightly golden.

Remove from oven, let rest for 5 minutes to set. Garnish with fresh basil or parsley, then serve warm.