Aioli - In a small bowl, mix the mayonnaise, capers, lemon juice and zest and the fresh dill and refrigerate until ready to use.
Cucumber Olive Topping - Use pre-chopped olives and give yourself a break. Make sure to chop the onions fine, and as for cucumber, I just use a regular cucumber for this (not seedless), but I do peel and remove the seeds. Chop the cucumber fairly fine and toss with the red onion, black olives, and pepper and refrigerate until ready to use. Right before you are ready to make the sandwich, season well with salt and pepper. But go light on the salt as the olives are salty.
Salmon - This is perfect for leftover salmon. But I wouldn't use leftover salmon if it was cooked with a strong sauce or which had completely different flavors, but a simple grilled or baked salmon would work great for this. But here is an quick easy method to cook the salmon which will work perfect for this dish.
Pan Seared/Steamed Salmon Recipe: In a small sauce pan over medium heat, add, 1/4 water and 1 teaspoon olive oil. Bring to medium heat and add the salmon fillets. Cook on medium flesh side down first for 2 minutes and then flip.Cook another 3-4 minutes depending on the thickness until fork tender, remove from the water to a plate to cool. No need to add flavors to the water because you will be seasoning with lots of other flavors. Once it is cool, lightly break it up into large pieces.
Sandwich - Very easy, just slather the first piece of bread with the aioli, spinach, then some of the flaked salmon, the cucumber and olive topping, tomato slice, then the second slice of bread slathered with the aioli. That's it.
Serve - Enjoy with some potato chips or potato salad and a cold beverage for a great lunch or light dinner. ENJOY!