Stuffed Mushrooms

Name: Stuffed Mushrooms
Ingredients: 2 teaspoons extra-virgin olive oil; 12 large white mushrooms, brushed clean, stems removed and reserved; 1/2 cup baby spinach, finely chopped; 1/2 onion, minced; 2 cloves garlic, chopped; 1/2 cup cherry tomatoes, finely chopped; 2 tablespoons flat-leaf parsley, chopped; 1/8 teaspoon salt, plus additional for sprinkling on mushroom caps; Pinch freshly ground black pepper; 3 tablespoons grated Pecorino Romano cheese; 3 tablespoons panko (Japanese breadcrumbs)