Brush the tuna with a little oil. Heat a non-stick pan, add the tuna and cook for 1 min each side, or a min or so longer for a thicker fillet. Transfer to a plate to rest.
Halve and stone the avocado and scoop the flesh into a small bowl. Add the mustard powder and vinegar, then mash well so that the mixture is smooth like mayonnaise. Stir in the capers. Spoon into two small dishes and put on serving plates with the lettuce leaves, and tomatoes.
Slice the tuna (it should be slightly pink inside) and arrange on the plates. Spoon some ‘mayo’ on the lettuce leaves and top with tuna and cherry tomatoes and a few extra capers. To eat, roll up into little wraps.