1/4 cup olive oil
4 medium yellow or sweet onions, chopped
1 large clove garlic, minced
1 tablespoon Worcestershire sauce
2 sprigs fresh thyme (optional)
2 cups plain Greek-style yogurt
Salt and pepper
Heat a large skillet to medium and add oil and onions. Stir onions to coat with the olive oil and season with salt. Sauté, stirring every few minutes, for 30 minutes or until the onions are a golden color. Add garlic and cook for one minute more. Add Worcestershire and thyme leaves (if using) and stir gently to combine. Remove from heat and let onions cool until they are barely warm.
Fold yogurt and onion mixture together in a bowl. Cover and chill for at least an hour before serving.