In a bowl, whisk together mayonnaise, mustards, garlic, chopped capers, lemon juice, lemon zest, and Old Bay Seasoning. Season to taste with freshly ground black pepper.
Refrigerate for 30 minutes to allow flavors to meld.
Garnish with a sprinkle of Old Bay and chopped chives or parsley, if desired.
Notes
Make ahead: this sauce actually improves after sitting a few hours or overnight in the fridge.Storage: airtight container in the fridge, up to 5 days.Not recommended for freezing, as mayonnaise-based sauces can separate once thawed.Grate the garlic rather than mincing it for the smoothest texture.