Heat 1 tablespoon of oil in a medium skillet over low heat.

Add the onion, cover, and cook until tender (not browned), about 8 minutes.

Stir in the tomato and continue cooking, uncovered, for about 10 minutes.

Let it cool slightly, then wipe the skillet clean.

In a small bowl, lightly beat the eggs with salt, pepper, and the flaked tuna.

Fold in the cooked onion and tomato mixture.

Heat the remaining tablespoon of oil in the skillet over high heat until hot.

Quickly pour in the egg and tuna mixture, spreading it evenly.

Reduce the heat to medium-high.

Cook, shaking the skillet often to prevent sticking, until the edges and bottom start to brown, and the center is mostly set.

Use a spatula to carefully flip the omelet to the other side.

Add more oil to the skillet if needed.

Continue cooking until the center is fully set but slightly moist.