In a small bowl, stir together ½ cup extra-virgin olive oil, 1 cup lightly packed fresh flat-leaf parsley (finely chopped), ¼ medium red onion (minced), 2 tablespoons red wine vinegar, ½ teaspoon red pepper flakes and ¼ teaspoon kosher salt. Taste and season with additional vinegar and salt, if needed. Use immediately or refrigerate in an airtight container for up to 3 days.