cream of mushroom recipe

4 tablespoons unsalted butter - 1 medium onion, finely chopped - 2 cloves garlic, minced - 1 pound fresh mushrooms, sliced (use a mix of your favorites like button, cremini, or shiitake) - 1/4 cup all-purpose flour - 4 cups chicken or vegetable broth - 1 cup heavy cream or half-and-half - 1/2 teaspoon dried thyme - Salt and pepper to taste - Optional: fresh parsley or chives for garnish