Preheat the oven to 350 degrees. Butter or oil a 2-quart baking dish. Combine the polenta, salt and water in the baking dish. Place in the oven on a baking sheet. Bake 50 minutes. Stir in the butter, and bake for another 10 to 15 minutes until the polenta is soft and all of the liquid has been absorbed. Stir in the cheese, if using.
While the polenta is baking, cook the vegetables. Heat the oil in a large, heavy nonstick skillet over medium heat. Add the onion and cook, stirring, until it begins to soften, about 3 minutes. Add the carrots, kohlrabi and parsnip, and then season with salt. Cook, stirring often, until tender, about 10 minutes. Add the garlic, and stir together for about a minute until fragrant. Stir in the tomatoes with their liquid, a pinch of sugar and salt to taste. Cook over medium heat, stirring often, for 15 minutes until the tomatoes are cooked down and fragrant. Add lots of freshly ground pepper, taste and adjust salt, and remove from the heat.
Serve the polenta with the vegetables spooned on top.