Trim and cut the 2 1/2 pounds of skinless chicken into bite-size pieces and place in a large mixing bowl.
Add 2 1/2 tsp sea salt, 1/2 tsp black pepper, 2 tsp red chili powder, 1 tsp ground coriander, 1/2 tsp turmeric, 1/2 tsp kasoori methi, 1 tbsp + 1 tsp garlic paste, 1 tbsp ginger paste, 1/2 cup whole milk yogurt, and juice of 1 lemon to the bowl with the chicken; mix thoroughly to coat.
Cover and refrigerate the marinated chicken for 1 hour.
Meanwhile, heat a large rondeau or deep skillet over low–medium heat and add 2 tablespoons unsalted butter; add the 2 thinly sliced red onions and caramelize, stirring every 5–6 minutes, until well browned (about 30 minutes).
Stir in 1 tbsp ginger paste and 1 tbsp garlic paste and cook until fragrant, about 30–45 seconds.
Add 1 tbsp garam masala, 1/2 tsp red chili powder, 1 tsp kasoori methi, the dried chilies, and the seeded chopped jalapeño; cook 1–2 minutes, adding a splash of water if needed to deglaze the pan and loosen browned bits.
Add the 10 cored, roughly cut Roma tomatoes and 1/2 cup cashews; stir, cover, and simmer on low–medium heat for 20 minutes, stirring every 5–7 minutes.
Transfer the tomato-cashew mixture to a blender and blend on high until smooth; if needed to thin, add about 1 cup cold water and blend briefly.
Strain the blended sauce back into the pot and keep over low heat; whisk in 1 stick (cubed) of cold unsalted butter until incorporated, then stir in 1/4 cup heavy whipping cream; season with salt to taste and keep warm.
In a separate large rondeau or frying pan, heat 3 tablespoons ghee or oil over medium–high until it begins to shimmer; add the marinated chicken in a single layer (work in batches if necessary) and sear 3–4 minutes without stirring, then stir-fry 1–2 minutes more until cooked through.
Transfer the cooked chicken into the warm sauce, stir to combine, then finish by mixing in 2 tablespoons butter, the remaining 1/2 teaspoon kasoori methi, and 2 tablespoons chopped fresh cilantro.
Adjust seasoning if needed and serve immediately.
