Heat the butter and oil together in a 2-quart saucepan. Saute' the onions and mushrooms until the onions are transparent.
Gradually stir in the whole wheat flour, stirring constantly so that it-coats the vegetables. Cook this mixture over gentle heat for about one minute, stirring all the while.
Add the milk, one cup at a time. Simmer the mixture until it has thickened slightly. Add the salt and cayenne while you simmer.
Stir up the ricotta until it is separated and soft. You can even put it in the blender if you're up to it, but it isn't necessary. Add the cheese to the sauce and use a whisk to break up any lumps. The sauce is ready when it's all hot.