Toss the chicken chunks with garam masala, Kashmiri chili powder, turmeric, black pepper, lemon juice, and salt until evenly coated. Cover and marinate for at least 15 minutes at room temperature while you prep the remaining ingredients.
Heat 2 tablespoons of oil in a large skillet or wok over medium-high heat until shimmering. Sear the marinated chicken pieces in a single layer for 3-4 minutes per side until golden and cooked through (165°F/74°C internally), then transfer to a plate and set aside.
In the same pan, add the remaining 1 tablespoon of oil over high heat. Add the slit green chilies, chunked onion, and bell peppers, and stir-fry for 3-4 minutes until slightly charred but still crisp, then transfer to the plate with the chicken.
Reduce heat to medium, add the finely diced onion, and cook for 5-6 minutes until soft. Add the minced garlic and grated ginger and cook for 1-2 minutes, then add the cumin, coriander, turmeric, and Kashmiri chili powder along with 2 tablespoons of water and stir for 1-2 minutes until fragrant.
Add the chopped tomatoes, tomato paste, and salt, then cook over medium heat for 8-10 minutes, stirring occasionally, until the tomatoes break down into a jammy sauce and the oil begins to separate at the edges.
Return the seared chicken to the pan, stir to coat in the sauce, and cook for 3-4 minutes. Add the stir-fried peppers and onions, sprinkle with the finishing garam masala, stir gently, and cook for 2-3 more minutes until everything is heated through and the sauce coats the chicken and vegetables.
Turn off the heat, squeeze a small amount of fresh lemon juice over the top, and garnish with roughly chopped cilantro and optional julienned ginger. Serve immediately with basmati rice, naan, or chapati.