Heat 2 tablespoons of olive oil in a large skillet over medium heat. Season catfish fillets with salt and pepper. Fry fillets in the hot oil, in batches if necessary, for 2 to 3 minutes on each side or until fish flakes easily when tested with a fork. Place fillets on a plate to cool, and drizzle with lemon juice.
Drain giardiniera, and place in a small bowl. Add olives, parsley, garlic and the remaining tablespoon of olive oil, and mix until combined.
Slice the bread in half horizontally, and pull out most of the soft white part in the middle. Spread half of the giardiniera mixture in the hollow of the bottom piece of bread. Arrange the fillets in a single layer over the vegetables. Top with layers of tomato slices, arugula leaves and provolone slices, if using, and remaining giardiniera mixture. Place the other half of the bread on top, pressing firmly into place.
Using a serrated knife, cut the sandwich into wedges, and serve. The wedges can be individually wrapped and served at a picnic.