Blanch the greens: Bring 4 cups of water to a rolling boil in a large pot with the baking soda. Add the thawed frozen spinach (and mustard greens if using) and blanch for 2-3 minutes until bright green. Drain immediately and rinse under cold water to stop the cooking.
Blend until silky: Squeeze the blanched greens well to remove excess water, then blend with 1/4 cup of fresh water until completely smooth and silky. Set the puree aside.
Sear the chicken: Heat 1 tablespoon of ghee or oil in a large, heavy skillet or Dutch oven over medium-high heat. Add the chicken pieces in a single layer and sear for 3-4 minutes per side until golden at the edges. Remove and set aside.
Build the masala base: In the same pan, heat the remaining tablespoon of ghee. Add the cumin seeds and let them sizzle for 20-30 seconds until fragrant. Stir in the chopped onion and cook for 6-8 minutes until soft and golden. Add the ginger-garlic paste and green chili; cook 1 minute more. Stir in the chopped tomatoes and tomato paste and cook 5-6 minutes until the tomatoes break down and the oil begins to separate at the edges.
Combine and simmer: Stir in the coriander, half the garam masala, turmeric, Kashmiri red chili powder, and salt. Pour in the blended spinach puree and 1 cup of water or chicken stock, and stir well. Return the seared chicken to the pan with any accumulated juices. Bring to a gentle simmer, then cover and cook for 15-20 minutes until the chicken is cooked through and the gravy thickens to coat a spoon.
Finish and serve: Uncover and stir in the heavy cream, the remaining 1/2 teaspoon garam masala, crushed kasuri methi, butter, and lemon juice. Simmer 2 more minutes, taste, and adjust salt. Drizzle with a swirl of cream, sprinkle with fresh cilantro, and serve hot.