Cut eggplant into 2cm chunks. Thread eggplant and mushrooms onto 8 wooden or metal skewers. Place vegetable skewers in a nonporous shallow dish and spoon the marinade over everything. Cover and refrigerate for at least 2 hours.
To make marinade: Mix together cumin, coriander, ginger and 2 cloves crushed garlic in a large frypan or wok. Stir over high heat until fragrant. Remove from heat then stir in soy sauce, oil and lemon juice.
To make peanut sauce: In a food processor grind cumin seeds, coriander seeds and 3 cloves garlic. Combine with onion, lemon juice, salt, chilli powder, coconut milk and peanut butter in a small saucepan. Stir in water. Simmer over medium low heat until it becomes a thick sauce. Transfer peanut sauce to a serving bowl.
Preheat barbecue to medium heat and lightly oil grate.
Remove skewers from marinade and place on the barbecue. Cook for about 10 to 15 minutes or until vegetables are tender. Serve immediately with peanut sauce.