I perhaps made something similar for Christmas last year? Although it was probably with wine rather than port.
From Bon Appetit, slightly modified.
1/4 c olive oil
1 large red onion, sliced thin
Salt and pepper
1/2 c ruby port
65 g light brown sugar
1/4 c red wine vinegar
Cook the onions in olive oil in a small saucepan over medium heat until softened, 8 minutes. Add remaining ingredients and summer for about 2 hours. Cool and cover.
This turned out good but I was unhappy that the oil never seemed to fully incorporate. Need to go back and find that other recipe from last fall!! [Found it, from Thanksgiving 2012]