Chop the rhubarb into wafer thin slices. Stir 1 tbsp caster sugar and ½ tsp salt with 2 tbsp vinegar in a large bowl to dissolve it. Add the rhubarb and toss to mix. Set aside to pickle.
Peel and finely chop the onion. Peel the garlic and chop it. Rinse the buckwheat in a sieve.
Chop the chicken into bite-sized chunks. Warm a frying pan for 1 min. Add ½ tbsp olive oil and the chicken. Season with salt and pepper. Fry for 5 mins, stirring now and then, till golden. Lift out of the pan and pop on a plate.
Add another ½ tbsp oil to the pan and add the onion. Stir and fry over a medium heat for 5 mins. While the onion fries, roughly chop the mushrooms. Chop the dill, leaves and stalks, but keep them separate.
Add the mushrooms, garlic and dill stalks to the pan and cook for 3 mins to soften. Season with pepper. Stir in the drained buckwheat. Crumble in the stock cube. Pour in 325 ml boiling water. Cover, bring to the boil and then simmer for 8-10 mins. Pop the chicken pieces back into the pan halfway through.
When the buckwheat is tender, check the chicken is completely cooked through with no pink meat. Divide the risotto between 2 shallow bowls. Scatter the dill fronds over the risotto. Serve with the pickled rhubarb.