Mix the chilli, garlic, capers, parsley, lemon juice and oil in a small bowl. Put the tuna in a shallow dish and toss well with the marinade. Season and then chill for 1 hour.
Heat the barbecue and grill the fish on direct heat for 2-3 minutes each side, depending on the thickness of your fish. Serve with green beans and crispy potatoes, if you like.
For more healthy BBQ recipes, see Healthy recipes.