Spinach and Artichoke Stuffed Portabello Mushrooms with Tomato and Bread Salad

Name: Spinach and Artichoke Stuffed Portabello Mushrooms with Tomato and Bread Salad
Ingredients: 8 portabello mushroom caps; Extra-virgin olive oil, for brushing mushrooms, plus 1/4 cup; 1 (10-ounce) box frozen chopped spinach, defrosted; 1 can quartered artichoke hearts, drained well; 1 1/2 cups ricotta cheese; 1 egg yolk; 1 clove garlic, grated with microplane or small side box grater; Salt and freshly ground black pepper; Freshly grated nutmeg, about 1/8 teaspoon; 1/2 cup Parmigiano-Reggiano, a couple of handfuls; 6 plum tomatoes, coarsely chopped; 1 cup fresh basil leaves, about 20, torn; 4 cups crusty bread, torn or coarsely chopped; 1 small red onion, very thinly sliced