Pre-heat the oven to 180 Fan/200C/Gas 6
Melt the butter in a small saucepan on low heat.
Cut the onions in half keeping the stalks intact & just trim a little off both ends.
Place the onions cut side up in a roasting tin or dish.
Pour the butter & marmite mixture over the onions cover the dish with foil & place in the oven.

After 45 minutes remove the foil from the dish & return it to the oven for a further 20 minutes.
Serve hot.