Melt the butter in a pan and simmer the onions over medium heat for about 5 minutes. Add the sugar and caramelize the onions a little. Add the vinegar and simmer gently. Season to taste with salt and pepper.
Put the onion jam on a starter spoon or Chinese soup spoon. Put the crumbled Stilton on top and finish each with half a slice of ham.
Can be served both cold and warm.
Tip: The raspberry vinegar can be mixed with fruit juice, e.g., pineapple juice or orange juice or be replaced by red wine.