Polenta Gratin With Spinach And Wild Mushrooms

Name: Polenta Gratin With Spinach And Wild Mushrooms
Ingredients: 2 tablespoons extra-virgin olive oil; 12 ounces sliced mixed wild mushrooms (5 cups); 1 large shallot, minced; 8 ounces prewashed baby spinach (8 lightly packed cups); 1/2 teaspoon thyme; Pinch of freshly grated nutmeg; Salt and freshly ground pepper; 1 tablespoon unsalted butter; 1 tablespoon all-purpose flour; 3/4 cup chicken stock or low-sodium broth; 1/2 cup cream; One 18-ounce log of prepared polenta, cut into 1/4-inch slices; 3 ounces Gruyère cheese, shredded (1 cup)