Put the tomato paste in a bowl. Add the salt, sugar, garam masala, cumin, cayenne, cilantro, green chili, lemon juice, (if using dried curry leaves, add them at this point - fresh go in with the stir fry), and 1 T water.
Mix well. Slowly add the coconut milk or cream, mixing as you go. Set aside.
The sauce can be made ahead and refrigerated for several hours.
Put oil in wok or skillet and set over medium high heat. When oil is hot, put in the mustard seeds. As soon as they begin to pop - a few seconds - put in the garlic and fresh curry leaves, if using.
Stir till garlic turns medium brown and put in the shrimp.
stir till the shrimp turns opaque most of the way through.
Add the sauce, turn the heat to medium and heat sauce through till it begins to simmer. The shrimp should be completely opaque and cooked thorugh. Turn off heat.