Preheat oven to 400℉ and spray a 9×13 inch baking dish with cooking spray.
Start by cooking potatoes and cauliflower in a pot of water until fork tender. Drain and add the butter, milk and sour cream. Use a potato masher to mash smooth. Fold in the cheese and season to taste with salt and pepper. Set aside until ready to use.
Meanwhile heat a large skillet to medium heat and add oil. Crumble and cook ground turkey and seasonings until turkey is no longer pink. Transfer meat to a plate and set aside.
In the same pan melt butter and cook onions and celery until just softened, about 3-minutes. Stir in mushrooms and garlic, and cook until mushrooms are soft and browned and the liquid has cooked out.
Next add flour and cook 30-seconds, then stir in tomato paste, Worcestershire sauce, chicken broth, frozen veggies, lentils, and add cooked turkey back to the pan. Combine and bring to a low simmer and cook until sauce is thickened, about 5-minutes.
Taste for seasonings then transfer mixture to your prepared casserole dish.
Top the filling mixture with the prepared potato cauliflower mash and smooth out the top with the back of a spoon.
Place dish in your preheated oven and bake for 30-35 minutes or until hot and bubbly and the top is slightly golden.
Garnish with fresh or dried parsley and allow to rest for 10-minutes before serving.
