Step 1:
In a small bowl, combine warm water, cilantro, lemon juice, turmeric, garam masala, ½ teaspoon salt, and green chile. Stir until well mixed.
Step 2:
Heat 2 tablespoons of oil in a 10-inch skillet over medium-low heat. Add the mixture from the small bowl to the skillet, cooking and stirring for 2-3 minutes. Add the shrimp and cook until they turn pink and are nearly cooked through, about 2-4 minutes.
Step 3:
Using a slotted spoon, transfer the shrimp to a bowl, leaving the sauce behind in the skillet. Pour 1 cup of water into the skillet to deglaze, scraping up any browned bits from the pan. Turn off the heat and set the sauce aside.
Step 4:
In a 3 to 4-quart Dutch oven or large pot, heat the remaining 2 tablespoons of oil over medium heat. Add the sliced onion and sauté until the edges begin to turn golden brown, about 3-4 minutes. Add the rice, 2 cups of water, 1 teaspoon of salt, and the reserved liquid from the skillet. Stir, bring to a boil, then cover and reduce the heat to very low. Let the rice cook for 25 minutes.
Step 5:
Once the rice is cooked, remove the lid and gently fluff with a fork. Stir in the shrimp, then cover and cook for an additional 10 minutes until both the rice and shrimp are fully tender. Stir in the butter until melted and the rice is evenly coated. Taste and adjust the seasoning with more salt or lemon juice as necessary. Garnish with additional cilantro and serve immediately.