Oddly enough, I'm not going to start out with a white sauce recipe of any kind; rather, I am going to go the soup route. More specifically, Seafood Soup.
Many much mushrooms, usually about half a pack
Half a stick of butter
Pack of imitation crab meat
A somewhat indeterminate amount of 2% or whole milk (preferably whole, but 2% works okay too)
Ground red cayenne pepper (if it happens to be lying around)
Large can of cream of mushroom soup
Sour cream (again, if it's lying around, if not, leave it out)
Put the onions and butter together to fry in a deep saucepan on low-medium heat; remember, it may seem like a lot of butter, but it's to help the whole soup later on, so trust me. Add about a teaspoon of salt, and the mushrooms after the onions have cooked somewhat, and add the crabmeat last. Fry until it reaches the level of golden brown you want.
Now add the entire can of cream of mushroom soup and make sure to stir it into the fry mixture well. Add about a cup of milk at this point to help even out the mixture and keep it from sticking to the bottom of the pan. Stir well using a whisk. Add a heaping spoonful of sour cream for flavor, stir well again. Add milk again until you reach the desired thickness (or thinness, or whatever). The amount really is dictated by the consistency you want. Make sure that every time you add milk you allow the soup to heat back up completely, since the milk added is cold.
Let the whole mixture simmer for about ten minutes while stirring every minute or so. I am obsessive about this stage and about making the consistency right, but don't mind my obsessions. Taste and add more salt (if needed) and pepper and red pepper if desired. Let it simmer for a couple more minutes, then serve.
If you do it right, this soup is really amazing as a leftover item to reheat. It can be a soup, and then after it has been refrigerated it can be resurrected as a white sauce over noodles or something. I even used a variant of it over hot toast (which I loved but my wife hated). Oh well. To each his (or her) own.
P.S. The dark stuff in the center of the soup in the picture is sage. Somehow it just tastes amazing when added right before you eat it!