1. Preheat barbecue to a high heat. Place couscous in a bowl, pour over boiling water, cover and set aside to stand for 5 minutes or until water is absorbed. Add butter and toss gently with a fork.
2. Heat oil in a frying pan over a medium heat, add onion and garlic and cook, stirring, for 3 minutes or until onion is soft. Add garam masala and cayenne pepper and cook for 1 minute longer. Add onion mixture to couscous and toss to combine.
3. Fill mushrooms with couscous mixture, top with feta cheese and cook on lightly oiled barbecue grill for 5 minutes or until mushrooms are tender and cheese melts.