Preheat oven to 190°C/375°F/gas mark 5. Line a baking tray with parchment paper. To make the marinade, in a large bowl, whisk together orange juice, lime juice, mayonnaise, syrup, tamari, garlic, onion powder, cumin, salt, and pepper. Use a food processor to chop the jackfruit. Alternatively, chop the jackfruit with a sharp knife. Depending on the brand of jackfruit, you may have to cut out hard centres of the jackfruit pieces and discard before chopping. Add chopped jackfruit to the bowl with the marinade. Mix well. Spread the jackfruit on the lined baking tray and bake for about 30 minutes until edges start to brown slightly.
To assemble the sandwiches, split bread and spread mustard liberally on both sides. Layer the sandwich with cheese, ham, pickles, jackfruit, then cheese again.
To cook, melt about 2 tablespoons of the butter in a large pan (cast iron works well) over medium heat. Depending on the size of your pan, place 1 or 2 sandwiches in the pan and top with another heavy pan or pot then put something heavy in the top pan or pot to help press the sandwiches down as they cook: canned beans or tomatoes work well. Sandwiches should be pressed to about ½ their original thickness.
Cook for about 5 minutes per side, pressing down occasionally until the sandwiches have flattened, the bread is toasted, and the cheese has melted. Alternatively, use an electric panini grill. Cut in half and serve immediately.