Mix the mustard, vinegar, maple syrup, oil and water. Grate in a little lemon zest, squeeze out the juice, add to the dressing, season.
Cook the buckwheat in salted water for approx. 3 mins. Remove the pan from the heat, cover and leave to absorb for approx. 15 mins.
Separate the buckwheat with a fork, mix with half of the salad dressing while still warm.
Peel and dice the sausage, gently fry in a non-stick frying pan until crispy, without adding any oil.
Add the seeds and cook for a further 5 mins. Drain the topping on kitchen paper.
Rinse and drain the sweetcorn. Slice the radish and dice the celery. Roughly chop the rocket. Mix it all together with the buckwheat, add the remainder of the dressing and the topping just before serving.