Can be made up to 2 days ahead. Heat oven to 120C/fan 100C/gas ½. Halve the tomatoes widthways and place, cut-side up, in a roasting pan. Peel the garlic cloves and slice very thinly into slivers. Put a sliver on each tomato, season and scatter over thyme or rosemary leaves. Sprinkle each with sherry vinegar, then with balsamic vinegar and finally with olive oil. Bake for 2-3 hrs until soft and slightly shrivelled. Remove and cool.