Using a peeler, create stripes down the length of the English cucumber, leaving some of the peel intact.
Cut the cucumber in half crosswise, then in half again lengthwise. Use a teaspoon to remove the seeds. Set aside.
Combine the shrimp, red onion, parsley, tzatziki and tomato in a small mixing bowl, then season with salt and pepper. Spoon the shrimp salad mixture into one cucumber half and use a second half to top. Use toothpicks to hold the top to the base. Repeat with the remaining filling and cucumber halves.