Step by step

Blanch the onions lightly in salted water, just so as to soften them a little

Tips
If you want to transform a recipe like this to one perfect for vegans, replace the butter with to olive oil and the cheese with one suitable for vegans.
Trivia
If you want to cut your onions without making yourself cry, try this cunning idea: wet the blade of your knife with some water before starting work.