Crossroads of the Mediterranean since Antiquity! 
My simple approach to tuna sandwich:
Ingredients:
1 can of Genova brand yellowfin tuna in olive oil w most of the oil squeezed out.
1 of those mini cans of diced green chilis, hot or mild as you prefer. Do include the liquid.
2-3 dill pickle chips diced small like the chilis.
1 teaspoon-ish of onion powder.
0, 1, or 2 slices of ordinary sandwich-size wheat bread.
Good cheddar cheese enough to make one bread-sized layer about 3/16" thick. For the usual bricks, maybe 4 slices.
Prep:
Very lightly toast the wheat bread. Like just barely colored and slightly stiffened. Limp is wrong. Cool is wrong. Crunchy is wrong.
Blend the tuna, chilis, pickles, and onion powder together, then add mayo (I use Best Foods or Hellmans depending on where I’m living 
) a tablespoon or so at a time & keep mushing it around until the right consistency. Which is still pretty meaty & dry, not smooth and unctious. But it does all stick together, no tuna slivers falling out. Probably 2 TBSp, maybe 2-1/2. Too little mayo is better than too much.
If skipping the bread, just eat the yummy tuna mash right out of the bowl. Using the same fork you made it with. With bites of cheese in between. Look Ma: nearly no dirty dishes!
Otherwise build the sandwich with bottom bread, cheese, tuna, optional top bread. You’ll probably have about 1/3rd of the tuna not fit on the sandwich. That’s OK; we fix. Inhale sandwich. Eat remaining tuna from the bowl. Repeat if necessary, but once I passed about age 40, one whole can of tuna + fixins has been plenty.
1 pouch tuna in water
A dollop of mayo
A dollop of Miracle Whip
A big glop of relish
A couple of grinds of black pepper
Mix thoroughly then spread on white toast or stiff a tomato.
Tuna to me is comfort food, so I make it the way my mother did.