Open the can of tuna and drain it thoroughly, pressing out as much excess liquid as possible to prevent the filling from becoming watery. Transfer the drained tuna to a medium mixing bowl and gently break it apart with a fork until the flakes are evenly separated.

Add the plain Greek yogurt, Dijon mustard, fresh lemon juice, a pinch of salt, and freshly ground black pepper to the tuna. Mix everything together until the ingredients are fully combined and the filling becomes smooth and creamy. Taste and adjust the seasoning if needed, adding a little more lemon juice for extra brightness if desired.

Wash the cucumber thoroughly under cold running water and pat it completely dry with paper towels or a clean kitchen towel. Slice the cucumber into even rounds about ¼-inch thick so each slice is sturdy enough to hold the tuna mixture without breaking.

Arrange the cucumber slices in a single layer on a serving plate or platter, leaving a little space between each slice for easy assembly and serving.

Using a spoon or a small cookie scoop, place an even portion of the prepared tuna mixture onto the center of each cucumber slice. Gently shape the filling with the back of the spoon to create a neat presentation while ensuring the topping stays securely on the cucumber.

Garnish each cucumber bite with freshly chopped dill, parsley, sliced green onions, or a light sprinkle of everything bagel seasoning if desired. Finish with a small squeeze of fresh lemon juice and a pinch of freshly cracked black pepper for extra flavor.

Serve immediately for the freshest texture, or refrigerate the tuna mixture separately and assemble the cucumber bites just before serving. If preparing ahead, store the cucumber slices and filling in separate airtight containers to keep the cucumbers crisp and refreshing.