Finely dice the onion and mince the garlic. Dice the tomatoes, skin, seeds, and all. Peel and slice the carrots ¾inch thick. Peel and cut the turnips into ¾-inch chunks. Break the head of cauliflower into small florets.
Heat the oil in a medium-size saucepan over medium heat. Add the onion and stir-fry for a minute to get it cooking. Reduce the heat to low and simmer covered until somewhat softened, about 3 minutes.
Stir in the spices and tomatoes thoroughly, then add the remaining ingredients. Simmer the curry covered over low heat until all the vegetables are tender, 30 to 35 minutes. Check the curry after 10 minutes of cooking; if the mixture seems dry and the vegetables don’t seem to be getting tender, add about ¼cup water. When the vegetables are done, season to taste with salt and pepper and serve hot.