Preheat oven to 350 degrees. Using a tablespoon of butter, grease eight 8-ounce ramekins and set aside.
Heat oil in a large sauté pan over high heat until hot. Add mushrooms and garlic and sauté 4 minutes.
Add sherry and wine, cooking until mixture is almost completely evaporated.
Add remaining butter and season with salt and pepper. Remove from heat and cool.
Combine sweet potatoes with brown sugar, cream, egg yolks and thyme. Mix well and season with salt and pepper. Whip egg whites on high speed of mixer until soft peaks form. Gently fold whites into sweet potato mixture.
Divide mushroom mixture among ramekins. Top with sweet potato mixture. Bake about 30 minutes or until a knife inserted in the center comes out clean. Serve warm.