Preheat the oven to 163° C.
Trim about 1cm off the top of each tomato and discard. Use a little spoon or a melon baller to remove the seeds and most of the flesh, leaving a clean shell.
Lightly salt the inside of the tomatoes and place upside down in a colander, to drain off some moisture.
Meanwhile, put the onion, garlic, olives, and 1 tablespoon of the oil in a medium pan and cook on low heat for 5 to 6 minutes, to soften the onion completely.
Remove from the heat and stir in the breadcrumbs, herbs, capers, and some pepper. Taste and add salt, if you like.
Wipe the insides of the tomatoes with a paper towel, then fill them up with the herb stuffing, pressing down very gently as you go to get a nice dome of stuffing on top.
Place the tomatoes in a greased ovenproof dish and drizzle lightly with the remaining oil. Bake for 35 to 45 minutes, or until the tomatoes soften. Serve hot or warm.
Recipe adapted from Plenty by Yotam Ottolenghi.