Cut the chicken into bite-size pieces and place in a large bowl.
Add 2 1/2 teaspoons sea salt, 1/2 teaspoon black pepper, 2 teaspoons red chili powder, 1 teaspoon ground coriander, 1/2 teaspoon turmeric, 1/2 teaspoon kasoori methi, the garlic paste (total amount listed in ingredients), 1 tablespoon ginger paste, 1/2 cup yogurt, and the juice of 1 lemon to the chicken; mix thoroughly to coat.
Cover and refrigerate the chicken to marinate for 1 hour.
While the chicken marinates, heat 2 tablespoons unsalted butter in a large rondeau or heavy pot over low–medium heat and caramelize the thinly sliced red onions, stirring every 5–6 minutes, until deeply browned, about 30 minutes.
Add 1 tablespoon ginger paste and 1 tablespoon garlic paste to the browned onions and cook 30–45 seconds until fragrant.
Stir in 1 tablespoon garam masala, 1/2 teaspoon red chili powder (sauce), 1 teaspoon kasoori methi, the dried chiles, and the chopped jalapeño; cook 1–2 minutes, adding a splash of water if needed to deglaze and loosen browned bits.
Add the 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; remove center piece of blender lid or cover with a towel to release steam safely.
Strain the blended sauce back into the pot through a fine-mesh strainer, return to low heat, and whisk in 1 stick (cubed) cold unsalted butter until incorporated.
Stir in 1/4 cup heavy whipping cream and adjust seasoning with salt; keep the sauce warm on low heat. If sauce is too thick, thin by blending with about 1 cup cold water and straining or by adding small amounts of water until desired consistency.
Heat 3 tablespoons ghee or oil in a large frying pan or second rondeau over medium–high heat until it just begins to smoke lightly.
Add the marinated chicken in a single layer (work in batches if needed so pieces do not touch) and cook undisturbed 3–4 minutes, then stir-fry 1–2 minutes until cooked through and lightly seared.
Transfer the cooked chicken into the warm sauce and stir to combine; finish by mixing in the remaining 1/2 teaspoon kasoori methi, 2 tablespoons butter, and 2 tablespoons chopped fresh cilantro.
Keep warm briefly and serve.