Day After Turkey Shepherds Pie

Name: Day-After Turkey Shepherd's Pie
Ingredients: 3 tablespoons olive oil; 1 large shallot, minced; 2 cloves garlic, minced; 1 cup diced celery; 8 oz crimini mushrooms, trimmed and sliced; 3 tablespoons flour; 2 1/2 cups shredded leftover turkey; 1 cup turkey stock; Kosher salt, to taste; Freshly ground black pepper, to taste; 2 tablespoons tomato paste; 1 teaspoon fresh thyme; Leftover mashed potatoes; 1/2 cup cheddar cheese; 1/4 cup parmesan cheese