Make a mayonnaise by placing in a blending jar the egg, the lemon, the salt and oil and blend with a hand blender until the mixture will look uniform and will have thickened.
Drain the tuna and add it in the jar along with the rest of the ingredients. Blend until smooth.
Serve as a dip or as a sauce, decorate with capers.
Store the sauce in an airtight container for up to 3-4 days in the refrigerator.
Notes
*In case you don’t find anchovies in your area, you can omit them and add more capers and mustard to balance the flavour.