Put water and vegan butter or margarine in a saucepan and heat, stirring, until the butter is melted.
Salt the water, add buckwheat and cook according to package instructions.
Peel onion and chop it. Rasp carrot coarsely. Cut white cabbage into thin strips.
In a deep frying pan, fry the onion in vegetable oil until translucent.
Add the grated carrot and fry, stirring constantly, for about 1 minute.
Add the white cabbage, season it with salt, black pepper, stir it and steam it with the lid on for about 20 minutes until the vegetables are just cooked.
Add the stewed white cabbage to the buckwheat in the saucepan, heat it for about 3 minutes while stirring, seasoning it with salt and pepper if necessary, and take it off the heat.