How to Make It
In a medium bowl, mix the bell peppers with the onion, canola oil, vinegar and a generous pinch each of salt and pepper. Let stand at room temperature for 1 hour.
Meanwhile, in a saucepan, cover the potato with water and bring to a boil. Add a generous pinch of salt and simmer over moderate heat until tender, 20 to 25 minutes. Drain and cool; peel and thinly slice.
Spread each toast with mayonnaise. Top the toasts with the potato, flaked tuna, pepper confetti and cornichons. Serve.