On a dry skillet, roast the sooji, stirring constantly, until it turns just a couple of shades darker and smells fragrant. Always roast on medium to medium-low heat because you don't want to burn it, and don't leave the stove unattended. The sooji should turn just a couple of shades darker and smell fragrant. Once it's roasted, remove to a plate and set aside.
Heat the oil in the same skillet. Add the mustard and wait for the mustard seeds to sputter.
Immediately add the curry leaves, green chillies and onions. Saute until the onions are soft but not brown. Add the ginger and stir in.
Add any additional veggies at this point, if using. Add the turmeric and saute another minute.
Add 4 cups water and salt it until it's saltier than you want the upma to be.
Turn the heat to low and have a ladle or whisk ready.
Pour the roasted rava into the skillet and immediately start stirring it into the water. You need to work fast at this stage, and again, make sure the heat is at the lowest point possible. You want every grain of the sooji to absorb water so the upma is nice and fluffy and not lumpy. If some lumps do form, press on them with the ladle and stir them into the rest of the upma. You can sprinkle on some water if needed.
Turn off the heat immediately and mix in the cilantro, lemon juice, and coconut, if using. Serve in bowls with more wedges of lemon that you can spritz over the upma.