Pre-heat the grill to hot.
Place the quinoa in a large, heavy-based saucepan. Heat over a moderate heat, stirring continuously until the grains separate. Add 600ml water and a little salt and bring to the boil. Reduce to a simmer and cook for 15 minutes or until the liquid has been absorbed.
Transfer to a mixing bowl and allow to cool. Stir in the avocado and parsley until combined. Whisk together the lemon juice, garlic and olive oil in a mixing bowl and dress the quinoa with it.
Meanwhile, scoop out the insides of the tomatoes and reserve the hollow tomatoes to one side. Arrange the tops and the hollow tomatoes on a lined baking tray, season, and grill for 3-4 minutes until the skins start to wrinkle.
Remove, then fill with the quinoa mixture. Return the tops to the grill for a few minutes to blacken on one side. Serve the stuffed tomatoes and tops on a serving plate.