Freezer Fix Shepherds Pie

Name: Freezer Fix Shepherd's Pie
Ingredients: Nonstick cooking spray, for the baking dish; 3 tablespoons unsalted butter; 1 onion, diced; 1 teaspoon dried sage; 1 teaspoon dried thyme; 1/4 cup all-purpose flour; One 16-ounce bag frozen peas and carrots, thawed; 2 1/2 cups low-sodium chicken broth; 1 tablespoon Dijon mustard; Kosher salt and freshly ground black pepper; 3 cups leftover shredded braised pork or rotisserie chicken; 4 cups leftover or frozen mashed potatoes, thawed; 3 tablespoons grated Parmesan