Heat a wide skillet. Add the rava and roast, stirring constantly, over medium heat until the rava turns a couple of shades darker. Remove to a plate and set aside.
In the same skillet, heat the oil. Add the cashews and the raisins, if using, and saute until they turn just lightly golden.
Add the soymilk and mango pulp and 1/2 cup of sugar. Stir and allow the mixture to come to a boil. Check for sweetness and add more sugar if needed.
Turn down the heat to low and add the vanilla and then the roasted rava. Using a whisk or a ladle, stir quickly to mix with the liquid. You need to work fast to ensure your halwa is not lumpy and that the rava and the soymilk-mango pulp mixture is smoothly integrated.
Turn off the heat when the halwa has a creamy consistency. It will continue to thicken as it stands, so don't wait until it's too dry.