Using serrated knife, cut top off bread and reserve. Cut out inside of bread, leaving 3/4-inch shell. Place bread and top on cookie sheet. Bake until lightIy toasted, about 10 minutes. Keep bread warm.
Meanwhile, melt butter with oil in heavy medium skillet over medium heat. Add both onions and saute until golden brown, about 10 minutes. Add mushrooms and cook until tender, about 10 more minutes.
Reduce heat to low. Add cheese and stir until thoroughly blended, about 4 minutes (do not boil). Pour Mixture into loaf. Replace top and serve, cutting base and top into wedges.