Combine the yoghurt and lemon balm, season. Cover and chill the dressing until ready to serve.
Cut a lid off each tomato, set aside. Scoop out the inside of the tomatoes, leaving a border of approx. 5 mm, transfer the insides to a bowl. Add the couscous, lemon juice and lemon balm, mix, pour boiling water over the top, cover and leave to absorb for approx. 5 mins. Add the peanuts, fluff up the couscous with a fork and stuff the tomatoes with the couscous mixture. Place the lids back on top of the tomatoes.
Place the tomatoes in a grill tray.
Charcoal/gas/electric grill: Grill the stuffed tomatoes over/on a medium heat (approx. 200 °C for approx. 15 mins. Serve with the yoghurt dressing.