In a small bowl, mix together the cumin, turmeric, chili powder, coriander, and garam masala. Set the spice blend aside.
Heat 1 tablespoon of oil in a large skillet over medium heat. Add the chicken, sprinkle with half of the spice mixture and a pinch of salt, and cook, stirring occasionally, for 7-8 minutes or until the chicken is fully cooked and reaches an internal temperature of 165°F (74°C). Transfer the chicken to a plate and keep it warm.
In the same skillet, add the remaining oil along with the onion and bell peppers. Sauté for about 2 minutes, stirring occasionally. Then, add the garlic, ginger, and chili, and cook for another 2 minutes until fragrant.
Deglaze the pan with a splash of water, then add the diced tomatoes, a pinch of sugar if desired, tomato paste, and the remaining spice mixture.
Stir the sauce and let it simmer for 3-4 minutes. Add the chicken back to the skillet, stirring to warm it through. If the sauce starts to dry out, add a splash of water and cook for 1 more minute.
Serve the dish over basmati or turmeric rice with a side of fresh naan bread. Garnish with fresh cilantro leaves for a burst of color and flavor.