You could use yellow onions here, as well as sweet onions. If you want to reduce the alcohol content of the relish, gently boil the mixture after adding the port for a few minutes longer.
2 T olive oil
1 1/2 – 2 c chopped red onion
6 whole cloves
1/4 c port; or 1/4 c red wine mixed with 1/2 t brown sugar; or 1/4 c Madeira
1/2 t maple syrup, agave, or liquid honey
salt and pepper, to taste
1. Heat the olive oil in a sauce pan over medium heat. Add the onion and cook, stirring, until softened, about 5 minutes. Add the cloves, cover, reduce heat, and cook until the onion is caramelized a deep browny purple (but not burnt), 10 to 12 minutes longer.
2. Add the port and cook, stirring, for a minute. Add the maple syrup, stir, then remove the sauce pan from the heat. Fish out the cloves, season with salt and pepper, and let cool before serving.