Peel, wash and chop the celeriac into bite size pieces. Place in a roasting tin and drizzle with olive oil and dried or fresh thyme. Mix well.
Roast for 40 minutes or until soft, stirring occasionally.
Meanwhile place the buckwheat in a medium saucepan with the water. Bring to the boil, reduce the heat and simmer for 8-10 minutes until all the water has been absorbed.
Slice the onion. Heat the olive oil in a frying pan and sauté the onion until soft. Add the garlic, cook for a minute and then add the balsamic vinegar.
Add the onion mixture and the cooked buckwheat to the roasted celeriac, season to taste and return to the oven for five minutes to warm through.