Cut chicken fillets into quarters. In a glass or ceramic bowl combine chicken with 4 teaspoons vinegar or lemon juice and turn to coat. Set aside.
In a spice grinder, grind coriander seeds, cinnamon, cardamom and cloves to a powder. Place in a bowl and combine with turmeric, chili powder, paprika, nutmeg, mace, remaining vinegar or lemon juice. yogurt, garlic, ginger and oil, and mix well. Season with salt and add to chicken. Mix well. Cover and place in refrigerator to marinate for 30 minutes.
Preheat oven to 475 F (240C/Gas 9). Oil a shallow roasting pan and place chicken pieces in pan in a single layer. Bake, without turning, for 12 minutes. Remove from oven and set aside.
To make sauce : In a deghchi or largo frying pan, heat oil and butter mixture over medium-low heat. Add onions and 1 teaspoon salt and cook, uncovered, stirring occasionally until onions are dark golden brown, about 15-20 minutes. Add ginger and garlic and cook, stirring for 2 minutes. Add chili powder, turmeric and chilies, and cook for 1 minute. Add tomatoes and cook, uncovered, stirring often, until tomatoes are soft, about 5-10 minutes.
Add cream and butter to pan and cook, stirring until butter melts. Stir in chicken, honey and fenugreek, and cook, stirring often, until chicken is cooked through, about 5 minutes. Stir in cilantro. Table and add salt if necessary. Serve immediately.