In a medium sized pan, melt 1 tablespoon of butter on medium heat. Add chopped onion and pan fry until golden brown. Remove from heat and transfer the onions to a food processor. Blend the onions while slowly adding about a tablespoon of water until smooth. Transfer the onions back to the pan on low to medium heat.
While the onions are blending, add the pasta to a pot of salted boiling water and cook to instruction. Reserve a ½ cup of pasta water
Stir in the ginger garlic paste, tomato paste, and salt to taste into the onion paste. Cover the sauce pan with a lid until the butter starts separating a little on the sides and the tomato turns a darker color. Be sure to stir it every now and then. The sauce should be very thick at this point.
Add all the spices and mix together well. Stir in the cream, and then the ½ cup of hot pasta water. Simmer until butter starts to separate and the sauce thickens. Pour in your pasta. Once the sauce is incorporated, mix in 1 tablespoon of melted butter before plating the pasta and garnishing with cilantro.
Notes
A great alternative to using the spices is buying a readymade spice mixture of butter chicken or tikka masala from your local Indian grocery store.
If you don't have ginger garlic paste available, you can toss fresh garlic and ginger into a food processor with some water and blend until a paste forms. Ground ginger and garlic are also acceptable substitutes, but not preferred over fresh ginger and garlic.