In a large saucepan for which you have a lid, add the oil and heat up on medium heat. Once warm, add the mustard seeds to the hot oil and allow them to pop for a few second.
Next, add the finely chopped onions and fresh chillies to the pan; mix well, put the lid on top of the pan, and allow to cook until the onions are soft and their edges start to lightly brown. This will take about 3-5 minutes.
Once the onions are ready, add the chopped tomatoes to the saucepan, together with a little salt. Once more, mix well, put the lid on top, and allow to cook until the tomatoes soften on a medium-low heat, which will take again about 3-5 minutes.
Once the sauce is ready, add the marinated chicken to the saucepan and fry on medium-high flame for 5 minutes.
Once the oil separates from the spices, add ¾ cup of water [150-200 ml], mix well again, and cook the chicken. Allow the achari chicken to simmer on low heat for about 15 minutes [or until cooked].
Finally, add the julienned ginger and allow to simmer for a further couple of minutes.
Once ready, before serving, top with freshly chopped cilantro [fresh coriander leaves] and lemon juice.
Serve hot with simple pulao rice, jeera rice, or any Indian bread, such as phulka, naan bread, and parathas.