Cut the tuna into large chunks, place in a food processor and pulse until roughly minced. Transfer to a bowl.
Stir in the salad onions, Cajun seasoning and sesame seeds. Using damp hands, shape the tuna mixture firmly into six evenly-sized burgers.
Grill or barbecue the burgers for around 5-7 minutes on each side until they are just cooked through.
Meanwhile, cut the baguette into six pieces. Split open the bread and spread with mayonnaise. Top with cucumber slices, the tuna burgers and some watercress.