If you don't have hard-boiled eggs ready, gently lower 2 eggs into boiling water and cook for 10–12 minutes, then cool in ice water for 5 minutes. Peel and place in a medium bowl.
Add the drained tuna, Greek yogurt, and capers. Mash with a fork until combined but still a little chunky.
Add black pepper, salt, and olive oil. Mix, taste, and adjust.
Toast the bread until golden and crisp.
Divide the mixture between the two slices and spread evenly.
Drizzle with balsamic glaze and serve immediately. Add fresh herbs or extra capers if desired.