In a small bowl, whisk together the lemon juice or vinegar, Dijon mustard, and whole grain mustard.
Slowly drizzle in the olive oil while whisking until emulsified.
Stir in the shallot, capers or cornichons (if using), lemon zest, parsley, salt, and pepper.
Vinegar Tip: Red wine vinegar adds deeper, fruitier notes while white wine or champagne vinegar keeps it light and bright. Choose what suits your mood.