"This is a variation on a recipe from Ontario Turkey. We like our version much better. The difference is poultry seasoning, no wine, and starting with cooked leftover turkey. We usually make a double batch and it never lasts very long."
Heat oil and butter in a large Dutch oven over medium high heat; cook and stir potatoes, onion, carrots, celery, garlic, and rosemary until onion is translucent, about 10 minutes.
Dust turkey with flour; stir into vegetable mixture. Stir chicken stock, mushrooms, poultry seasoning, salt, and pepper into turkey mixture; bring to a boil. Reduce heat, cover, and simmer until vegetables are tender, about 20 minutes. Remove from heat; whisk cream into soup.
Optional: If you prefer a thicker stew, melt 3 tablespoons butter over medium heat, gradually whisk in 3 tablespoons flour and, stirring constantly, whisk in cream to create a thick cream sauce; pour into soup and continue to cook until thickened, about 3 minutes. Serve over Kaiser buns.