Heat the oven to 350 F.
Break the cauliflower into small pieces. In a large saucepan, steam the cauliflower, garlic and pieces of leek in water until completely tender — about 20 to 30 minutes.
Use a food processor to puree the vegetables until the texture is like that of mashed potatoes. Process only a small portion at a time.
If you prefer a smoother texture, use a blender. Be sure to hold the blender lid on firmly with a dish towel. Add a little hot water if the vegetables seem dry. Stir in margarine and pepper.
In a large saute pan, heat the oil over medium heat. Add the carrots, celery, onions, thyme and garlic. Saute until then vegetables soften, about 5-6 minutes. Add the turkey and cook until browned, about 2-3 minutes. Add the red wine and cook until the wine is reduced by half. Add the vinegar and tomato sauce. Season with pepper.
Transfer the mixture to a large casserole dish. Top with the mashed cauliflower. Bake for 20-30 minutes, until the top of the casserole is golden brown.
Remove from the oven and let rest for 5 minutes. Top with minced chives and orange zest, if you wish, and serve.