Heat the oven to 350 F.
Break cauliflower into small pieces. In a large saucepan, steam cauliflower, garlic and leeks in water until completely tender, about 20 to 30 minutes. Use a food processor to puree the vegetables until the texture resembles 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 vegetables seem dry. Stir in margarine and pepper.
In a large saute pan heat oil over medium heat. Add carrots, celery, onions, thyme and garlic, and saute until vegetables soften, about 5-6 minutes. Add turkey and cook until browned, 2-3 minutes. Add red wine and cook until wine is reduced by half. Add vinegar and tomato sauce. Season with pepper.
Transfer mixture to a large casserole dish. Top with the mashed cauliflower. Bake for 20-30 minutes until top of casserole is golden brown.
Remove from oven and let rest for 5 minutes. Top with minced chives, if you wish, and serve.