Preheat oven to 350 degrees F. Lightly coat an 8×8 baking dish with nonstick spray.
In a medium bowl, whisk together butter, poppy seeds, garlic and mustard powder and salt.
Top one side of the rolls with arugula, tomatoes, ham and cheddar, and then the tops of the rolls. Repeat with remaining rolls to make 9 sandwiches.
Place rolls into prepared baking dish and spread the poppy seed dressing evenly over the top. Cover with aluminum foil and place into oven and bake until cheese has melted, about 10-15 minutes.