In a medium bowl combine Red Onion, Lemon Juice and Zest, Olive Oil from Sardines, Mustard, and Mayonnaise. Mix until combined.
Fold in celery and Sardines break up sardines.
Fold in fresh Herbs and top with Pepper Flake if using.
Serve on bread or toast for a sandwich. Can also be served on crackers or atop a fresh salad.
