In a large non-stick skillet or pan heat the oil on medium heat for about 1 minute. Add in the onions and sauté well for 4 minutes, stirring occasionally until the onions soften, wilt and begin the turn golden at the edges.
Add in the ginger paste and the garlic and cook for another 2 minutes.
Add in the cumin and coriander powders and mix well.
Place the entire mixture in a slow cooker, add in the salt, sugar and the tomato and turn the setting on a high.
Let this cook undisturbed for 1 hour on high and add in the chicken and cook for another 45 minutes, until the mixture is bubbly and cooking briskly.
Place the sour cream and cashews in a blender and grind until smooth. Then, stir into the chicken and let it simmer for 15 to 20 minutes.
Place the mint in a blender, squeeze in the lime juice and add the cilantro and green chilies and grind to a paste.
At this point, the chicken should be soft and the gravy should be a light creamy texture.
Turn off the slow cooker and stir in the herb sauce and mix well.
Let rest for 20 minutes before serving.