Preheat oven to 160°C/325°F/Gas Mark 3. Sit tomatoes on a board, stem-side down, and cut the top off, about ½ cm(¼ in) deep. Finely dice the top. Scoop inside seeds out with a small teaspoon and push these through a sieve to extract juice or simply discard them if you can’t really be bothered – and on a hot day even I might be pushed to do this!
Fry chicken and onion in sesame oil over a moderate heat until almost cooked. Remove from heat and add diced tomato tops, Thai fish sauce, mint, half the spring onions and peanuts. Mix well, then spoon mixture back inside tomatoes.
Lightly oil a roasting dish and sit tomatoes in, cut sides facing up. Bake for 15 minutes, at which point the skins should almost be bursting, but not quite. Remove from oven and carefully place on a plate. Mix any tomato juices with lemon juice and olive oil and spoon over the tomatoes. Scatter the coriander and the remaining spring onions on the tomatoes and grind on some pepper.