Stuffed Portobello Mushrooms with Spinach and Goat Cheese

Name: Stuffed Portobello Mushrooms with Spinach and Goat Cheese
Ingredients: 10 portobello mushrooms (each 4 to 5 inches), stems removed and reserved, caps wiped clean; 4 tablespoons olive oil; Salt; 12 ounces baby spinach (about 10 cups); 2 tablespoons water; 2 large slices white sandwich bread , torn into quarters; 2 tablespoons unsalted butter; 2 medium onions , diced small (about 2 cups); 4 medium garlic cloves , minced or pressed through garlic press (about 4 teaspoons); 1/2 cup dry sherry; 2 tablespoons chopped fresh thyme leaves; 4 ounces goat cheese , crumbled (about 1 cup), see note; 1/4 cup heavy cream; 1 cup walnuts , toasted and roughly chopped; 2 teaspoons fresh lemon juice; Ground black pepper