Brush both sides of whole portabellas with olive oil, season lightly with salt and pepper and grill approximately 10 minutes per side.  Melt butter in medium fry pan. Add sliced button mushrooms and sherry and cook for about 8 to 10 minutes over medium heat.  Season ground chuck with salt and pepper, portion into 8-ounce patties and grill to desired doneness (recommended 160°).  To assemble, top each portabella with a burger, mushroom slices and grated Gouda cheese. Move to the top shelf of grill and close lid until cheese is soft and slightly melted.