Preheat the broiler. Mist a foil-lined broiler pan with cooking spray. Put the mushrooms gill-side down on the pan; broil 5 minutes.
Meanwhile, toss the spinach, breadcrumbs, mozzarella, chopped shallot, tomato, 2 tablespoons each pecorino and olive oil, 1/2 teaspoon salt, and pepper to taste in a medium bowl.
Remove the mushrooms from the broiler and turn them gill-side up on the pan. Season with salt and pepper. Mound the spinach filling on the mushrooms and top with the ricotta. Sprinkle with the remaining 1 tablespoon pecorino and broil until the filling is heated through and the ricotta is lightly browned, about 5 minutes.
Meanwhile, toss the celery, sliced shallot and the remaining 1 tablespoon olive oil in a bowl and season with salt and pepper. Top with shaved pecorino. Divide the mushrooms among plates and serve with the celery salad. Drizzle with olive oil.
Photograph by Antonis Achilleos
Courtesy of Food Network Magazine