Start by cutting off the ends of the zucchinis. Put them in a vertical position and slice a thin strip to make it as flat as possible, to serve as a base.
Now put 2 skewers on each side of the zucchini. With a knife, cut it into round slices, until you get to the skewers. As soon as you get to them, stop cutting, to prevent you from cutting all the way through.
Repeat this process with the rest of the zucchinis.
Season the zucchinis with salt - open them and add 1 pinch of salt to each slice. Set them aside and let them rest for 15 minutes.
Cut the tomatoes into round slices and cut these slices in half.
Cut the mozzarella cheese into medium slices.
Get a slice of tomato and a slice of cheese, put them together and place them into one of the zucchini slits.
Repeat this process until you've filled all slits on the zucchini.
Transfer these stuffed zucchinis to a baking dish, season with black pepper and oregano.
Bake in a preheated oven at 200°C (392°F) for 40 to 60 minutes.
Serve.