Stuffed-Shells Florentine

box ((12-oz) Jumbo Shells)

olive oil ((I used Star Grapeseed Oil))

to 5 cups sliced button mushrooms ((about 2 8-ounce packages))

garlic cloves (, minced)

((14-oz each) stewed diced tomatoes)

bags ((16-oz each) fresh baby spinach leaves)

salt and fresh ground pepper (, to taste)

tomato sauce (, divided)

part-skim shredded mozzarella cheese