Wash the cucumber and slice it into thick, round slices—these will be your ‘toasts’.
In a bowl, mix the drained tuna with mayonnaise or yogurt, lemon juice, and chopped shallot.
Season with salt and pepper to your liking.
Spoon a little tuna mixture onto each cucumber slice.
Top with fresh dill or parsley for garnish.
Serve immediately for best texture and flavor.