Preheat oven to 400 degrees F.
Make the topping: place cut up cauliflower florets (or peeled and cut potatoes) in a pot of water with 3 cloves of garlic and 1 teaspoon salt. Boil for 30 minutes (or until fork tender) and drain on a colander.
Transfer cauliflower or potatoes back to the pot, add milk and sour cream and blend until smooth with an immersion blender. You can also use a potato masher - see Note below.
Mix with cheddar cheese, taste and season with salt and pepper, to taste.
To make the topping: in a large skillet heat the olive oil, then brown the ground turkey with ¾ teaspoon salt, ½ teaspoon pepper, ½ teaspoon thyme and ½ teaspoon paprika.
When browned, transfer to a plate and set aside.
Add 1 tablespoon butter to the empty skillet and cook onions and celery until softened, about 5 minutes.
Add chopped mushrooms and minced garlic. Cook together until mushrooms are softened, 3-5 minutes more. Then add flour and mix well.
Add tomato paste, frozen veggies, cooked turkey, Worcestershire sauce and chicken broth. Mix and cook on low until thickened, about 10 minutes.
In a casserole dish (I used a ceramic 9 x 12 dish but any dish such as Pyrex or a cast iron pan will work) spread the turke-veggie mixture in one even layer.
Place mashed cauliflower or potatoes on top in "blobs", then spread with the back of a spoon so it covered the meat and veg.
Bake in 400 degrees for 30 minutes or until bubbly. If you want, you can turn the broiler on for a few more minutes to crisp up the top.
Let cool for a few minutes and serve sprinkled with chopped parsley, if desired.