Preheat the oven to 200˚C. Spread four to five heaped tablespoons of canned tomato over a piadina, salt generously, and sprinkle with oregano. Top with cooked ham, a mixture of Emmentaler and Gruyere cheese, and olives. Place another piadina on top. Bake in the oven for 10 to 15 minutes, until the cheese is melted and the piadina is slightly crispy around the edges.