For the mayonnaise:
1) In a small bowl, mix together the mayonnaise and sweet gherkins until smooth.
For the sandwiches:
1) Place a griddle pan over medium-high heat. Season the tuna with salt and pepper. Drizzle with the oil and sprinkle with the herbs de Provence. Griddle for 2 minutes on each side for medium-rare. Place a slice of cheese on each piece of fish during the last minute of cooking. Using a pastry brush, brush the bottom half of the bread with 4 tablespoons of olive oil. Cut into 4 pieces, each about 10cm long and 8cm wide. Griddle, cut side down, for 30 seconds to 1 minute until toasted and golden.
2) Spread the griddled bread with the sweet pickle mayonnaise. Place a tuna steak on top. Top with the rocket and drizzle with olive oil. Season with salt and pepper, to taste, and serve.
* Cook's Note: Make the leftover top half of the ciabatta into croutons by cutting into 2-cm pieces, drizzling with olive oil and baking for 10 to 12 minutes in a preheated 190 degrees C/Gas 5 oven. Alternatively, tear the bread into small pieces, allow to dry, and blend in a food processor to make bread crumbs.