In a bowl place 50ml of the olive oil, the tomatoes, capers, basil and marjoram. Add the mozzarella along with the garlic and lemon juice. Season with salt and cracked black pepper and gently combine. This can be done up to 3 hours in advance. Remove the tuna from the refrigerator an hour before cooking. Coat the tuna pieces in the remaining olive oil, then place on the hot chargrill of the barbecue for 20 seconds each side. Remove, slice in half and season. Divide the tuna and salad equally among the plates, then pour the juice that has accumulated in the bowl over the salad for dressing. SIMON SAYS: To store tuna, remove packaging, rinse fish under cold water and pat dry with paper towels. Fish deteriorates when it sits in its own juices, so place it on a cake rack in a shallow pan filled with crushed ice. Cover with cling wrap or foil and set in the coldest part of the refrigerator. Tuna will keep well this way for up to 2 days.