Cut the tomato into 1 cm wide slices crosswise, and chill in the refrigerator. Chop the onion and parsley finely.
Drain the oil from the tuna and mix with half the onion and the mayonnaise, and chill in the refrigerator.
Mix the mustard, vinegar, vegetable oil, salt and pepper and the remaining onion together, and chill until you're ready to serve.
Sandwich the tuna mixture from Step 2 between the tomato slices, add the parsley to the dressing from Step 3 and mix, and pour plenty over the tomato-tuna.
Story Behind this Recipe
I learned a recipe like this a long time ago in cooking school. It's an elegant little salad you can make with everyday ingredients.
Use great tasting tomatoes. If you aren't confident that you can slice your tomatoes cleanly, using a serrated bread will make it easier. The parsley will change color if you mix it into the dressing, so it's best to add it just before serving.