In a large heavy bottomed skillet (cast iron preferred), heat the oil over medium heat until hot but not smoking. Add the onions and chopped tomatoes and stir-fry until soft. Stir in the green chiles, ginger, garlic, peppercorns, chili powder and garam masala and cook for another 5 minutes to bring out the aroma of the spices.
Add the chicken (whole breasts or chopped) and cook for 5 minutes turning frequently. Stir in the chicken stock and cook until all the flavors have combined, about 10 to 12 minutes. Add the tomato paste and stir, which will help thicken the sauce. Remove from the heat and add the vanilla yogurt, cilantro and lemon juice. Season with salt and pepper and let it rest for another 5 minutes until all the flavors meld. Transfer to a serving bowl and serve with Basmati Rice and Naan Bread, if desired.