Saute cumin and mustard seeds for 30 seconds. Add ginger and onion, saute for 12 minutes. Puree tomatoes in blender. Mix in sour cream and mix well. Add salt, cayenne, fennel, turmeric and coriander to the onion mixture. Cook for 1 minute. Add tomato mixture, simmer for 2 minutes. Add chicken and simmer for 40 minutes. Mix in garam masala and serve over rice.