Tangy olives balance sweet onions and peppers in this salsa. Its tartness is a good alternative to a squeeze of lemon juice on different sorts of white fish, such as tilapia.
Put onions, butter, and 2 tablespoons water in a saucepan over medium-low heat. Cover; cook until onions cook through, about 6 minutes. Cool. Quarter onions. Combine with remaining ingredients in a bowl. Serve cold or at room temperature.
Refrigerate in an airtight container up to 3 days.