Peel, wash and mince the onions. Melt the butter in a saucepan, and brown the onions until they turn a slightly golden color. Pour in some of the bouillon. Season to taste, cover and simmer for 30 minutes. Remove the rind from the Saint-Nectaire and cut it into thin slices. Cut the bread into pieces. In a soup tureen, make alternating layers of cheese slices and pieces of bread. Finish with the cheese. Sprinkle with bouillon and bake under the oven grill so that the surface becomes crispy golden brown.