For the mayonnaise: In a small bowl, mix together the mayonnaise and sweet pickles until smooth. For the sandwiches: Place a grill pan over medium-high heat or preheat a gas or charcoal grill. Season the tuna with salt and pepper. Drizzle with the oil and sprinkle with the herbs de Provence. Grill for 3 to 4 minutes on each side for medium-rare. Place a slice of cheese on each piece of fish during the last minute of grilling. Using a pastry brush, brush the bottom half of the bread with 1/4 cup of olive oil. Cut into 4 pieces, each about 4-inches long and 3-inches wide. Grill, cut side down, for 30 seconds to 1 minute until toasted and golden.
Spread the grilled bread with the sweet pickle mayonnaise. Place a tuna steak on top. Top with the arugula and drizzle with olive oil. Season with salt and pepper, to taste, and serve.
Make the leftover top half of the ciabatta into croutons by cutting into 1-inch pieces, drizzling with olive oil and baking for 10 to 12 minutes in a preheated 375 degree F oven. Alternatively, tear the bread into small pieces, allow to dry, and blend in a food processor to make bread crumbs.