Creamy Mushroom Chestnut Soup

Name: Creamy Mushroom Chestnut Soup
Ingredients: 3 tablespoons butter, divided; 1 tablespoon olive oil; 1/2 cup diced shallots; 1 cup diced celery; 2 1/2 cups peeled, diced red bliss potatoes (about 3); 8 cups mixed mushrooms, cremini and white button (about 2 1/2 pounds); 4 cups mushroom broth; 8 ounces cooked chestnuts (about 20); 1/2 teaspoon ground cinnamon; 1/4 teaspoon ground nutmeg; 1 tablespoon fresh chopped rosemary; 1 tablespoon fresh chopped sage; 1/4 teaspoon sea salt; 1/4 teaspoon freshly ground black pepper; 1 1/2 cups cream; fresh sage, rosemary, and chopped chestnuts for optional garnish