Put the stock in a saucepan and bring to a simmer. Put the kettle on just in case. Heat the oil in a separate large saucepan. Finely chop the onion and fry gently in the oil. When it is translucent and softening, add the rice. Stir until all the grains are coated in oil, then add a ladleful of hot stock. Stir until all the liquid is absorbed, then add another ladle of stock, stirring all the while. Continue until half the stock is in, at which point add two ladles at once. Give it a quick stir, then leave it for a minute to chop the tomatoes into strips. So long as the heat is not too high, it won't burn or be ruined by a short pause in stirring. Once you've prepared the tomatoes, return to stirring and adding the stock. When the rice is almost done (taste to check), add the tomatoes to the pan. Put the balsamic vinegar and sugar in a milk pan and heat to dissolve the sugar. Let it simmer very gently to reduce and thicken slightly. Add the spinach, roughly tearing up large leaves, to the risotto with the last ladle of stock (or hot water from the kettle if you run out of stock before the rice is cooked through). Crumble the goat's cheese in, stir well and season with pepper before dividing between two bowls. Pour a drizzle of the now sticky and sweet-sharp balsamic glaze over each puddle of risotto before serving.
*You won't need all of the glaze that results, but you need enough to cover the bottom of the pan to start with. You could use the remainder on roasted vegetables, or diluted in a salad dressing.