Arrange the tomato quarters around a large plate and spread the bell pepper, cucumber and onion slices in the middle of the plate.
2 bell peppers, 1 small onion, 2 large tomatoes, 1 small cucumber
Drain the tuna, flake it and place the flaked tuna on top of the bell peppers and onion on the plate.
2 cans tuna
Make a vinaigrette by combining together the olive oil and lemon juice along with the minced garlic and mustard powder. Season to taste with freshly ground black pepper. No need for extra salt here, the tuna is likely already salted.
6 teaspoon extra-virgin olive oil, 2 teaspoon lemon juice, 1 clove garlic, ⅛ teaspoon mustard powder, Sea salt and freshly ground black pepper to taste
Pour the vinaigrette over the tuna on the plate and sprinkle the chopped black olives on top as well.