Preheat oven to 425 degrees F.
Make the meatballs. Combine ground pork with nduja, panko, onion, parsley, salt, pepper, garlic, and egg, mixing until well combined. Shape mixture into 12 meatballs and place in a baking dish. Bake meatballs for 10 minutes. Remove from oven.
Reduce oven to 325 degrees F.
Pour marinara sauce over meatballs and bake for another 15 minutes. Remove from oven and let cool.
Make the pesto. In a food processor or blender, pulse together arugula with parmesan, walnuts, garlic, salt, pepper, and olive oil until well combined. Set aside.
Heat 2 tablespoons oil in a large saute pan over medium heat. Saute sliced bell pepper and onion until softened and browned, about 7-8 minutes. Remove from heat and let cool.
Increase oven to 350 degrees F.
Carefully slice dinner rolls in half down the middle, keeping the rolls intact. Place bottom half on a baking sheet and spread 3 tablespoons pesto followed by sauteed peppers and onion. Place one meatball per roll and top with sliced fontina. Place remaining bread on top.
Combine dried basil with oregano, parsley, and garlic powder in a small bowl. Brush melted butter on top of bread and sprinkle herb mixture. Bake for 15-20 minutes or until cheese is melted. Serve while hot.