Steam or boil the potatoes until tender. Drain and cool immediately in cold water.
Make dressing by combining all ingredients except oik in a food processor. While the machine is running, gradually pour the oil in a thin stream. (This will help the dressing to emulsify naturally without an egg.
Season the tuna with salt and pepper and a dash of Herbs de Provence. Grill for a couple of minutes on each side. the middle should still be pink or it will be dry.
bed of greens
Top with crumbled tuna and dressing