Preheat oven to 375 degrees F.
Cook pancetta in a large skillet on medium heat for about 5 minutes, until it starts to get crisp. Add the pine nuts and cook until they are toasted and the pancetta is brown and crispy, about 3 minutes more.
Add spinach to pan and cook while stirring for 2 minutes. Season with salt and pepper.
Place mushrooms on a baking sheet with the gill side facing up. Season with salt and pepper. Divide the tomato sauce among all the mushrooms, smoothing the sauce over the surface.
Divide the spinach mixture among the mushrooms on top of the sauce. Top each mushroom with mozzarella.
Bake for 25 minutes until the mushrooms are tender and the cheese starts to brown.
Let mushrooms cool for 10 minutes and cut each into quarters.