This is my own recipe:
3 onions, cut in half, remove outer layer, and cut into 1/4 in. slices
3 T. butter
6 c. beef stock or broth
1/2 t. salt, plus more to taste
freshly ground black pepper to taste
2 c. portobello mushrooms, cleaned and cut into 3/4 to 1 in. pieces
1/3 cup grated parmesan cheese
Melt the butter in a 4 qt. soup pot. Add the onions. Sautee for several minutes, then add the broth/stock and the 1/2 t. salt (how much salt you need for this recipe will depend on your broth/stock). Cook the soup on low for about an hour, or until onions are sweet and transparent.
Add the mushrooms. Cook for another 15 min. or so. Add the cheese, and black pepper.