Put the onions, sugar and vinegar into a pan on a low heat.
Cook slowly for 25mins until reduced and compote like and chill slightly.
Mix in the Orkney cheddar, creme fraiche, eggs, mustard and season with salt and pepper.
Pre-heat grill on a medium heat.
Cut the rye bread into squares or bite size circles and toast. Lightly spread with a little chutney and cover with a good amount of rarebit.
Grill slightly and serve.