To make the mayonnaise, put the garlic, egg, lemon juice and mustard into a food processor and blend. Keep the motor running while pouring in the oil and the Maggi seasoning. Add a little hot water if the mayonnaise is too thick.
Preheat a griddle. Drizzle a little oil over the steaks, then grill for one minute, turning once. Spread the mayonnaise over one half of the ciabatta roll. Top with the rocket, steak and onion slices. Close with the remaining half ciabatta and serve.