Wash, trim and slice the spring onions. Roughly chop the sage. Heat the oil and briefly fry both, then leave to cool. Wash the tomatoes, cut a lid from each one and hollow out.
Season inside with salt. Finely chop the tomato flesh. Drain the tuna and flake with a fork. Mix with the chopped tomato, spring onions, lemon juice and green peppercorns and fill the tomatoes with the mixture. Serve with fresh bread.