I figured tuna and carrot on their own would be boring and taste overmuch of virtue, so I used it as a starting point:

I mixed up:
grated carrot
finely chopped spring onion, celery and garlic
tuna (from a tin)
parsley, black pepper
mayonnaise (low-fat)
lemon juice
chilli sauce

and loaded it into a scraped multi-seeded muffin.

This is a tasty, refreshing type of sandwich. I can definitely recommend tuna as a sandwich filling.