1. Microwave butter in medium glass dish for 45 to 60 seconds, on HIGH, or until melted. Stir in garlic salt.
2. Preheat microwave browning grill for 5 minutes on HIGH.
3. Wash mushrooms; halve or slice each lengthwise. Stir into butter mixture, coating with butter.
4. Place mushrooms on preheated grill.
5. Microwave for 1 minute on HIGH. Turn mushrooms over and continue cooking for 1 to 2 minutes, on HIGH, or until tender.