Chicken with Porcini Gravy and Polenta

Name: Chicken with Porcini Gravy and Polenta
Ingredients: Handful dried porcini mushrooms (about 1/2 ounce); 3 cups chicken stock; 8 boneless, skinless chicken thighs, pounded; Salt and pepper; 2 tablespoons flour, plus more for dredging; 2 to 3 tablespoons olive oil; 4 tablespoons butter; 2 cloves garlic, finely chopped; 1 onion, finely chopped; A few fresh sage leaves, chopped; A few fresh thyme leaves, chopped; 1/2 cup dry white wine; 1 cup milk; 1 cup quick-cooking polenta; 1/2 cup grated Parmigiano-Reggiano; Finely chopped fresh parsley, for garnish