Preheat a barbecue plate or char-grill on medium-high heat.
Combine oil, garlic, chillies and thyme in a small jug. Brush over both sides of each mushroom. Season mushrooms with salt and pepper. Set aside for 5 minutes.
Place mushrooms flat-side down onto barbecue plate or char-grill and cook for 3-4 minutes on each side or until just tender.
Meanwhile, combine avocado, yoghurt and salt and pepper in a small bowl. Mix well.
Place mushrooms onto serving plates. Top with avocado mixture and beef. Serve immediately.