Heat the oil in a wide saucepan and add mustard seeds. When they sputter, add the cilantro, curry leaves if using, and garlic. Saute for a minute.
Add the onions and saute for a minute until they begin to turn translucent. Add the tomatoes and powdered spices– the garam masala, cayenne and turmeric. Saute, stirring frequently, until the tomatoes release most of their liquid and get pulpy.
Add the cashews, Brussels sprouts and some salt to taste. Cover the saucepan to let the Brussels sprouts get tender, around 5-8 minutes. Stir occasionally to make sure nothing's sticking to the bottom. If necessary, add a couple of tablespoons of water to the pan.
Add half the coconut milk and let the sauce come to a simmer. Check if the Brussels sprouts are tender by piercing one with a fork in the center. Add the remaining coconut milk and just warm through without letting the stew boil.
Check salt and add more if needed.
Serve the stew hot with some rice or bread. Squeeze on some lemon or lime juice over the stew while serving.