Put the buckwheat in a heavy frying pan and toast over a medium heat, stirring and tossing, for 4–5 minutes or until it has become slightly darker in colour. Remove from the heat.
Put the buckwheat in a non-stick saucepan, pour over the stock and water and bring to the boil. Cover and simmer for 5 minutes or until the buckwheat has absorbed all the stock.
Stir in half the oil, cover and cook for 10 minutes. Remove the lid and run a fork through the buckwheat, tossing and turning to separate the grains. Cook over a very low heat, uncovered, for a further hour, tossing the buckwheat with a fork every 10 minutes to separate the grains.
Meanwhile, make the dressing. Put all the ingredients in a bowl and whisk together until thoroughly mixed.
Heat the remaining oil in a wok or large frying pan. Add the seafood and stir-fry over a medium heat for 2–3 minutes or until hot. Add the hot seafood to the buckwheat, together with the cucumber, sugar snap peas, fennel and radishes. Drizzle over the dressing and toss gently to mix.