Preheat the grill to high. Arrange the tomatoes, onion and pepper on a baking tray, cut sides down, with the garlic cloves.
Grill the vegetables for 6 to 8 minutes or until the skins are beginning to blacken. Remove the skins from the onion and pepper, and squeeze the garlic flesh from the skins.
Purée the tomatoes whole with their skins, with the stock and garlic. Peel and chop the onion and red pepper, and stir into the soup. Stir in the tuna, green pesto and basil leaves, and simmer for 3 minutes.
Sprinkle each bowl with 1 tablespoon crumbled feta cheese and 1 teaspoon pine nuts to serve.