Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced onions and bell peppers, stir-frying until slightly softened but still crisp, then remove them from the pan and set aside.
In the same pan, add the ginger garlic paste and slit green chilies, sauteing for 1 minute until fragrant.
Add the chicken pieces, turmeric powder, and red chili powder, cooking until the chicken is browned on all sides.
Stir in the chopped tomatoes and salt to taste, cover the pan, and simmer for 10 to 15 minutes until the chicken is cooked through.
Return the sauteed onions and bell peppers to the skillet, sprinkle with garam masala, and cook for an additional 2 to 3 minutes.
Garnish with fresh chopped cilantro and serve hot alongside naan bread or basmati rice.