In a small bowl, combine the cumin, turmeric, chili powder, coriander, and garam masala. Set aside.
In a large skillet, heat 1 tablespoon of oil. Add the chicken, season with half of the spice mix and salt, and cook while stirring occasionally for 7-8 minutes or until the chicken is fully cooked (reached 165°F/74. Transfer to a warm, and keep warm.
In the same skillet, add the remaining oil, onion, and bell peppers. Cook for 2 minutes while stirring.
Add the garlic, ginger, and chilli. Cook for 2 more minutes.
Deglaze the pan water, then add the diced tomatoes, sugar if using, tomato paste, and the remaining spices.
Stir and simmer the sauce for 3-4 minutes, then add the chicken back in and warm it up. If you the sauce is drying out, just add a splash of water and cook for 1 more minute.
Serve over basmati rice or turmeric rice, and fresh naan bread. Garnish with fresh cilantro leaves.