In a large mixing bowl, combine one can of drained King Oscar's sardines, three tablespoons of mayonnaise, one tablespoon of stone ground mustard, thinly sliced scallions, and diced celery. Add capers, lemon zest, and juice from half a lemon. Mix thoroughly.

Gently flake the sardines with a fork while mixing. Aim for a cohesive but chunky texture.

Toast two slices of whole grain bread until golden brown and crispy, about 3-4 minutes.

Place one slice of toasted bread on a plate, spread the sardine salad mix over it. Optionally layer with lettuce, tomatoes, or red onions, then top with the second slice of toast.

Cut the sandwich in half diagonally and serve immediately while the bread is warm and crisp.