In a medium mixing bowl, whisk together the grapefruit juice, vinegar, mustard, honey, garlic, shallot and anchovy filllet, if using. Season with black pepper and a pinch of salt.
Whisking constantly, slowly stream in the olive oil and ensure the dressing is fully emulsified. Use immediately or refrigerate in an airtight container.
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Notes
If you have particularly sour grapefruits, go ahead and omit the vinegar.