Remove skin off the salmon fillet and cut it into bite sized pieces. In a medium bowl mix the yogurt and the tandoori spice masala mix and add the salmon pieces, gently stir to cover the salmon in marinade, refrigerate for 1 hour or at least 30 minutes.
In a large pan heat 1 tbsp of the butter or ghee, then add the salmon (reserve the marinade) and cook the fish until sealed all over but not cooked all the way through, which should take about 5 minutes. Remove to a separate plate and set aside till needed.
To the same pan add the remaining butter or ghee, chopped onion, ginger and garlic and saute over low heat for 10 minutes, then add ground coriander, turmeric, tomato paste, stir to combine, then deglaze the pan with water. Bring the reserved marinade to the mix and stir one more time, then turn the heat to medium and bring to a simmer, lower the heat and continue simmering for 5 minutes.
Now add the double (heavy) cream, stir, bring the salmon back to the pan and cook for 5-7 minutes longer. Add salt in needed. (Most tandoori masala mixes have salt already, so taste to check before adding salt.)
Serve with basmati rice, naan and fresh cilantro.