Peel the red onion and cut it into very small cubes. Sprinkle with salt and let it sit for 5 minutes.
Place the Greek yogurt into a mixing bowl. Add the canned tuna. Drizzle with the lemon juice.
Chop the olives finely and add them as well.
Season with salt and pepper, add the red onion, and mix everything thoroughly.
Let the spread rest in the refrigerator for at least 2 hours so the flavors can blend together.
Once ready, spread it onto whole grain bread. Serve with cucumber and lettuce on the side.