Tart onion and lingonberry relish

Thinly slice 1 red onion, mix with frozen lingonberries (airelles in French, or you could use cranberries), squeezed lemon and lime juice (about a table spoon of each), sugar and salt. Let sit at room temperature until the onions are pink, tossing along the way. Cover and refrigerate for up to 5 days. 

Good with fatty meats or heavy dishes to add a bit of tartness.