Blend the cashews with water until you have a very smooth paste.
Place the potatoes on a baking sheet sprayed with oil, toss them with the salt and chili powder, then spread in an even layer. Spray the tops once more with oil. Roast in a 350 degree oven for 20-30 minutes or until the potatoes are cooked and golden. Set aside.
Heat 1 tsp of oil in a saucepan. Add the cumin and coriander seeds and stir-fry until the coriander seeds darken a couple of shades.
Add half the onions, green chilies, ginger, and garlic. Season with some salt.
Saute the onions, stirring frequently, over medium-low heat, until they are softened and translucent.
Add the mint, coriander, and chard or spinach. Cook, stirring frequently, until the chard has wilted, about five minutes.
Place the chard-coriander-cumin-onion mixture in the blender, add enough water to make a thick paste, and blend until very smooth.
In the same saucepan, heat the remaining oil.
Add the mustard seeds. When they sputter, add the remaining onions.
Season with salt and saute until the onions become soft and translucent.
Add the chopped tomatoes and tomato paste or ketchup.
Cook, stirring frequently, until the tomatoes are reduced almost to a pulp.
Add the blended greens, garam masala, and the chickpeas. Stir well and let the mixture come to a boil. Turn down the heat and simmer for a couple of minutes.
Add the cashewnut paste and let the sauce heat through. Turn off the heat.
Add the lemon juice and salt if needed. Garnish with the roasted potatoes.
Serve hot with rice or rotis or naan.