How to make
Peel and grate the tomatoes on a fine grater. Mix them with a pinch of salt and about 2 tbsp. olive oil.
Cut the onion into thin strips and put the butter and the remaining olive oil on a low to moderate heat until it softens. When cooked over low heat, it releases its natural sugars and begins to caramelize even if we do not add sugar. But in this case, for a stronger contrast, add a spoon or two of brown sugar and a pinch of salt. Allow them to become golden and slightly crispy and remove the caramelized onions from the heat.
Spread one or both sides of the slices of bread with tomatoes and olive oil. Put the turkey fillet on one, spread caramelized onions, fresh salad leaves (lolo rosso, arugula, baby spinach, etc.) and a spoonful of light mayonnaise. Cover the products with the other slice and serve the delicious sandwich instantly.
In case you are going to eat the turkey and caramelized onion sandwich later, but want to prepare it in advance, then do not spread the bread with the tomatoes, but serve them separately in a small bowl for everyone to add before eating the sandwiches.
This is recommended so that the tomatoes do not soak the bread.
Enjoy your meal!