In a small bowl, let red onions sit in approximately 3 tbsp of dressing. Set aside.
Place red potatoes in a pot of salted water. Boil until fork tender (approximately 12 minutes).
Drain potatoes and cut in half. Place them in a bowl with 1/4 cup of the dressing.
At the same time the potatoes are cooking, add greens beans to a pot of salted boiling water. Cook until bright green, approximately 5 minutes.
Drain green beans and place in a bowl of ice water. Remove from water after a minute.
On a large serving platter arrange the butter lettuce. Place tuna in the middle of the lettuce. Arrange tomatoes, onions, potatoes, green beans , olives, capers and boiled eggs around the tuna.
Drizzle with remaining dressing and serve.