In a large, heavy-bottomed pan, heat the onion, celery and olive oil over medium heat until softened, about 10-15 minutes. Add the oat groats, stir to thoroughly coat and toast for about 5 minutes. Add the white wine and allow the oats to absorb the liquid, stirring occasionally, about 5 minutes.
Add the stock paste and water, bring to the boil, then reduce the heat to a simmer and cover. Leave the oats to cook and soften completely in the stock. This took me about an hour, but it could take a little longer, depending on your oats. There is no need for much more than the occasional stir during this time, however towards the end of the cooking time, as the risotto thickens, is is best to keep an eye on it and stir it more frequently to avoid the bottom catching and burning. If there is too much liquid, but you feel that the oats are done, remove the lid, increase the heat and boil off the excess. You will probably need to stir frequently during this time to stop it burning. If the mixture is very thick and you don't feel as though the oats are fully softened, add a little more water and continue cooking.
Turn off the heat and add the broad beans, yoghurt, Parmesan, mustard and thyme. Stir to combine well, cover again and set aside for at least 10 minutes while you prepare the toppings.