Bring onion, vinegar, 1/4 cup water, and sugar to a boil in a small saucepan over medium-high. Remove pan from heat; let stand 5 minutes. Drain.
Divide the avocado slices evenly among bread slices; mash with a fork. Sprinkle with 1/4 teaspoon salt and 1/4 teaspoon pepper. Top evenly with tuna, onions, parsley, almonds, and olives. Sprinkle with remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper.