In a medium saucepan bring 1 cup water and the salt to a boil. Add the couscous, stirring quickly. Remove from heat and cover. Let stand for 4 to 5 minutes. Fluff with a fork.
Meanwhile, cut off tops of tomatoes (and reserve for decoration, if you wish). Core and hollow out the tomatoes using a melon baller or spoon. Discard the core. Chop up the tomato pulp and mix, along with any juices, into the couscous.
Stir in the parsley, cilantro, mint, and feta cheese into the couscous mixture. Spoon into the prepared tomatoes.
Put the tomatoes in a baking pan and roast for 20 minutes, or until tomatoes are tender but not cracking. Remove and serve.