Food Network teaches how to crush, slice and mince garlic. Peel off some of the papery skin from the garlic and then smash the head of garlic with the heel of your hand to loosen the cloves; if you only need a few cloves, leave the head intact and pull some off. Separate the cloves. To peel a clove, cut away the root end with your knife. Lay the flat side of the knife over the clove while holding the knife handle, then with the heel of your free hand carefully whack the knife against the garlic to separate the skin from the clove. To crush the peeled garlic, lay the flat side of the knife over the clove and smash it again. To slice peeled garlic, lay the clove flat on the cutting board and hold it with the fingertips of one hand, keeping them curled under. Using a rocking motion with the knife, make thin slices by moving the knife slowly across the clove. To mince peeled garlic, lay the flat side of a knife over the clove and smash it. Roughly chop the clove then move your free hand flat across the tip of the knife and use a rocking motion to chop the garlic until it’s finely minced.
Preheat the oven to 325 degrees F. Brush a baking sheet with 1 teaspoon of the oil.
Finely chop the mushroom stems and set aside. Place the mushroom caps on the prepared baking sheet and bake for 12 minutes.
Meanwhile, heat the remaining 1 teaspoon oil in a medium skillet over medium heat. Add the mushroom stems, spinach, onion, and garlic and saute until the spinach wilts, about 6 minutes. Add the tomatoes and cook for 2 minutes more. Turn off the heat and stir in the parsley, salt, pepper and 2 tablespoons each of the pecorino and panko.
Preheat the broiler. Divide the spinach mixture evenly among the mushroom caps. Sprinkle each with the remaining 1 tablespoon pecorino and panko, then broil until golden brown, 2 to 3 minutes. Serve warm.