(From The Frugal Gourmet, p. 240-1)

Onions Marinated with Feta

2-3 sweet onions, peeled and sliced paper thin
1/4 pound feta cheese, crushed
Parsley or green pepper, chopped (optional)
1/4 cup olive oil
2 tablespoons white wine vinegar
2 tablespoons lemon juice
1/2 teaspoon dried oregano
Salt and pepper to taste
Sugar to taste (optional)

Place the onion slices and feta cheese in a bowl, along with the
parsley or green pepper if desired. Mix together the oil, vinegar,
lemon juice, oregano, salt and pepper. Add to onion and cheese, and
allow to marinate for 2 or 3 hours before serving. You may need a
pinch of sugar if the dressing is too tart for your taste.

This is a great side dish for practically any meal.