Heat the 2 teaspoons oil over medium heat in a small skillet. Add the onion slices and toss until they begin to soften but still maintain a little crunch. This takes out any bitterness and highlights the sweetness of the onion. Set aside to cool.

Flake tuna into a large bowl (big flakes) and add beans, capers, arugula, and cooked onions. Drizzle with 3 tablespoons olive oil (best quality) and 1 teaspoon red wine vinegar. Season with sea salt and freshly ground black pepper, then toss gently, so as not to break up the tuna pieces.

Plate and serve immediately.