Preheat oven to 180 °C.
Place the yeast, sugar and lukewarm milk in a bowl and mix to combine.
Set aside in a warm place for five minutes or until bubbles appear on the surface.
Place the flour, salt, oil and yeast mixture in a bowl and mix until a smooth dough forms.
Knead for five minutes or until smooth and elastic.
Prove until doubled in size, knock back and then shape into buns and place on a greased tray.
Heat the butter in a pan and cook the onions slowly until they are golden brown and soft. It should take about 15 minutes.
Top the buns with onion.
Leave to rise again and then bake for about 35 minutes.
Dunk these delicious buns into steaming bowls of soup.