In a medium saucepan, cook the onion in the olive oil over moderate heat, stirring, until softened, about 4 minutes. Add the cloves, cover and cook over low heat until the onion is deeply caramelized on the bottom, about 12 minutes. Add the port and cook, stirring, for 1 minute. Add the maple syrup and remove from the heat. Remove and discard the cloves. Season the relish with salt and pepper and let cool.
Make Ahead: The relish can be refrigerated for up to 1 week.