Heat the oil in a skillet over medium-low heat. Add the onion and cook, stirring frequently, until softened, about 8 minutes. Add the garlic and cook another 2 minutes. Allow the mixture to cool slightly.
In a blender, combine the feta, vinegar, lemon juice, mustard, olives, oregano, 1/2 teaspoon salt, some pepper and the onion mixture. Blend until smooth.
Pour enough dressing over the salad to coat the leaves and toss to combine. Save the remaining dressing for another use.
Use any remaining dressing to marinate chicken for a quick and easy dinner the next day.