Preheat oven to 350F.
Cook Pasta Shells in boiling water for 8 minutes, or until al dente. DO NOT overcook because they will tear and fall apart.
In a large skillet, melt 1/2 tablespoon butter over medium-high heat.
Add sliced mushrooms and cook for 5 minutes.
Stir in garlic; add remaining butter, and continue to cook for 1 minute.
Add in drained tomatoes.
Season with salt, pepper, Italian seasoning, and basil; bring to a simmer.
Stir in the spinach. Since we're using a lot of spinach, you will want to cook it in batches. As soon as one batch is cooked down, add the next - I break it up into 3 batches.
Remove from heat. Taste for seasonings and adjust accordingly.
Drain shells and rinse with cold water; pat dry with paper towel.
Spread 1-cup marinara sauce on the bottom of a 9x13 baking dish.
Stuff each shell with the spinach mixture and arrange over the marinara sauce in the baking dish.
Sprinkle the stuffed shells with shredded mozzarella cheese.
Bake for 20 to 25 minutes, or until bubbly.
Remove from oven.
Garnish with parsley and serve.