Put the buckwheat, water and stock cube in a pan. Bring to the boil and cook for about 15-20 minutes until the water has completely absorbed. Cover and rest for 10 minutes.
Meanwhile, thinly slice the red pepper and cut the onion into thin wedges.
Cut the tofu into 2cm cubes.
Put all in a baking dish, pour over the sesame oil, mirin, rice vinegar and soy sauce. Toss well, then bake at 180°C for 20 minutes. Alternatively, use an air fryer, set to 15 minutes.
When the pepper and onion pieces are tender, the tofu looks golden, transfer them to a serving bowl including any cooking jus from the pan.
Add the buckwheat and toss gently to combine.
Taste and adjust the seasoning to your liking.
Fold in the parsley and serve.