Cook the rice. To do this, pour cold water into a saucepan, put in the pre-soaked rice. Cover with a lid and put on medium heat. When the rice boils, reduce the heat. Cook for about 15 minutes until the rice is ready.
Prepare vinegar syrup for rice. Mix apple cider vinegar with sugar. Add vinegar syrup to cooked rice and cover with a lid. Let the rice cool for 10-15 minutes.
Prepare the tuna filling. Mix the flesh with mayonnaise until it reaches the consistency of a pate.
Make a sandwich. To do this, place a sheet of nori with the shiny side down and put 2 tablespoons of rice in the middle of the sheet. Smooth out the rice so that there is some space left from the edges. Form a sandwich in layers. Put spinach leaves, then put a layer of cucumber and pepper. Put 1 tablespoon of tuna on top, smooth it out with a spoon. Put the rice as the last layer. Fold the corners of the nori to the center to form an envelope. Make two more sandwiches in the same way.