Heat the vegetable oil in a large saucepan over medium-high heat.
Add the finely diced onion and sauté until it starts to soften and turn translucent.
Add the chopped garlic, ginger and green chili to the saucepan and cook for 1-2 minutes until fragrant.
Mix in the garam masala, ground cumin, ground coriander, and mild chili powder, stirring for 30 seconds until the spices are fragrant.
Add the chicken breast pieces to the pan and stir to coat with the spice mixture. Cook for 4-5 minutes, stirring occasionally, until the chicken is browned on all sides.
Add the tomato paste, diced tomatoes, and water to the pan, and stir to combine.
Reduce the heat to medium-low and simmer the curry, covered, for 10-15 minutes or until the chicken is cooked through and tender.
Taste and adjust the seasoning with salt, if needed.
Garnish with the roughly chopped fresh coriander and serve hot over your favorite rice or with naan bread.
