In a medium bowl, whisk olive oil, vinegar, lemon juice, dill, parsley, salt, and pepper.
Slice the cucumbers ¼-inch thick. If using an English cucumber, cut the slices into quarters.
Add the cucumbers and sliced onion to the bowl and toss to combine. Refrigerate for at least 30 minutes or up to 4 hours.
Stir and sprinkle with feta cheese before serving. Garnish with extra herbs if desired.
