In a medium size bowl, add tuna and break it up with a fork. Add cannellini beans and mix well, roughly mashing up some of the beans with the fork. Add spring onions, mayonnaise, salt, pepper, paprika and lemon juice and mix well. Allow to sit for 10 minutes for the flavours to meld whilst preparing the toasted bread.
In a hot oven, sandwich press or toaster, crisp up the bread.
Spread the tuna mix onto the toasted bread, top with lettuce, top with another slice of bread.