Chop the onion finely and fry till it is well softened, then add the minced garlic and fry for another minute or so. Add the cumin and mustard seeds and fry until they pop.
Chop up the tomato (or use canned), and add a little water and salt. Add the diced turnips and chopped turnip leaves. Heat on medium till it boils, then turn the heat to low, cover and cook (stirring occasionally) for about 10 minutes until the turnips are tender.