Blend the mayonnaise, parmesan and vinegar until very smooth, either in a blender or in a bowl.
Add the rest of the dressing ingredients, including a splash of water.
Rub the paprika and dried thyme onto the steak, season well, then rub with oil and barbecue for about 3 minutes on each side. Once cooked, slice up the meat.
Toss the baby gem leaves and red onion in the dressing, and then layer up the leaves, onion, croutons, and sliced meat onto a plate and scatter with parmesan shavings.
Notes
Best served immediately. If preparing to eat out of home, make sure all parts totally cool. For best results, keep all in separate pots and assemble together just before eating.