1/2 cup extra-virgin olive oil 2 medium onions -- halved and sliced thin 6 cloves garlic, sliced thin 1/2 cup black olives -- such as kalamatas, pitted and chopped coarse 1/4 cup capers (drained capers) 2 anchovy fillets -- minced 1/4 cup balsamic vinegar 1 teaspoon minced fresh marjoram or oregano leaves 2 tablespoons minced fresh parsley leaves salt and ground black pepper
Heat 2 tablespoons of oil in a large skillet. Add the onions and saute over medium heat until softened, about 5 minutes. Add the garlic and saute until fragrant, about 1 minute longer.
Turn the onion mixture into a medium bowl. Stir in the remaining ingredients, including salt (use sparingly) and pepper to taste.
Serve warm or at room temperature (The relish can be covered and set aside at room temperature for several hours).