Freezer Fix Shepherd's Pie

Nonstick cooking spray, for the baking dish

unsalted butter

onion, diced

dried sage

dried thyme

all-purpose flour

bag frozen peas and carrots, thawed

low-sodium chicken broth

Dijon mustard

Kosher salt and freshly ground black pepper

leftover shredded braised pork or rotisserie chicken

leftover or frozen mashed potatoes, thawed

grated Parmesan