Bring some water to a boil in a pan or steamer with a rack and lid. Layer the carrots, snow peas, and scallions evenly on rack. Place the halibut steaks in a single layer, on top of the vegetables. Sprinkle with salt and pepper. Cover and steam for about 5-7 minutes or until fish flakes easily. While fish is cooking, mix the butter and thyme together and set aside. When done, transfer the vegetables and fish to a warmed platter and pour butter and thyme mixture over. Garnish with lemon.