Heat 1 tablespoon olive oil in a 12-inch (30-cm) nonstick skillet over medium heat and sauté the onion until wilted, about 2 to 3 minutes. Add the pepper, tomato, tarragon and tuna. Reduce heat to low and cook for about 5 to 6 minutes, so that any juices from the vegetables evaporate. Remove skillet from the heat and set aside for about 5 minutes so that the mixture cools slightly. Wipe skillet clean.

Whisk the eggs, salt and pepper together. Add the tuna mixture to the eggs.

Heat the remaining 1 tablespoon of olive oil in the skillet over high heat and pour in the egg mixture. Lower heat to medium, tilt the skillet so that runoff from the egg is evenly spread over the entire skillet. As soon as the bottom is set and lightly brown, place a large plate over the skillet and, using an oven mitt to protect your hand from the heat, flip the tortilla over onto the plate. Slide it back into the skillet to cook on the other side for another 2 to 3 minutes. The tortilla should be juicy not dry. Do not overcook. Remove, and serve immediately.