Preheat oven to 400°F.

Place potatoes in a large saucepan and cover with water.

Bring to a boil, then reduce heat and simmer for 10 minutes, or until fork-tender.

Drain potatoes and transfer to a large bowl.

Add nonfat sour cream and reduced-calorie margarine.

Mash until smooth and season to taste with salt. Set aside.

Heat olive oil in a large skillet over medium-high heat.

Add chopped onion, diced carrots, and diced celery.

Cook until softened, about 3 minutes.

Add uncooked ground turkey breast and cook until browned, breaking up the meat, about 5 minutes.

Add all-purpose flour, fresh rosemary, dried thyme, salt, and black pepper.

Stir to coat the turkey mixture.

Add chicken broth and bring to a simmer.

Simmer until mixture thickens, about 3 minutes.

Transfer turkey mixture to a 9-inch deep-dish pie plate.

Spread mashed potatoes over the turkey mixture.

Using the back of a spoon, make decorative swirls on top of the potatoes.

Bake until potatoes are golden, about 30 minutes.

Slice into 6 pieces and serve.