In a saucepan with plenty of extra virgin olive oil, gently sauté the finely sliced Tropea onions. Add the apple cider vinegar, brown sugar, salt, some of the water, and a pinch of freshly ground pepper, spacing each addition by about 2 minutes. Let everything soften and reduce, then add the remaining water and turn off the heat. Blend the mixture until smooth, creating a jam-like cream with a perfectly balanced sweet and sour flavor. Cut 8 slices of rustic bread, each about 1 cm thick. Use a 10 cm ring mold to cut the bread into rounds. Lightly grill the bread in a non-stick pan with a drizzle of olive oil and a pinch of pepper until both sides are golden and slightly charred. Slice the Moliterno Truffle Cheese into 4–6 slices about 5 mm thick, then cut them into 10 cm rounds using the same ring mold. Set aside. Finely chop the beef with a knife, then place it in a bowl with olive oil, salt, and pepper. Let it marinate for about 10 minutes so the tartare develops a rich flavor. Assemble the snack in layers: start with a slice of bread, followed by a slice of Moliterno, a layer of beef tartare, another slice of Moliterno, and finally another slice of bread. Top with the Tropea onion cream and garnish with a mint leaf for a fresh finishing touch.