How to Make Mushroom Sauce
To make mushroom sauce, start by melting 2 tablespoons of butter in pan and adding 2 tablespoons of flour, ½ teaspoon of salt, and a pinch of pepper to the mix. As you stir the mixture, add 1 cup of milk and continue to stir until the sauce reaches your desired consistency. Next, melt 1 ½ tablespoons of butter in a separate skillet and brown chopped onions and mushrooms. Finally, add the mushrooms and onions to the white sauce and enjoy!
Melt 2 tablespoons butter or margarine. There are a couple different ways you can do this. You can put the butter in a microwave-safe container and melt it in the microwave ten seconds at a time, stirring it every ten seconds. The butter will melt very quickly this way, so don't let it burn. You can also melt the butter on the stove. Use a double broiler to melt the butter slowly. You need a bowl that fits over a small pot. Place the butter in the bowl. Bring water to boil in the small pot. Place the butter bowl over the boiling pot of water, and let the steam from the water slowly melt the butter. Stir the butter to make it melt more quickly. You can also melt the butter directly in the pan in which you'll make the medium white sauce. However you melt the butter, put the melted butter in the pan in which you'll make the medium white sauce. Add two tablespoons of flour, ½ teaspoon of salt, and black pepper to taste. Stir the mixture over medium heat until it's smooth. Make sure not to have the heat up so high that the flour burns — you want the mixture to come together slowly. Add 1 cup of milk. Pour it into the pan slowly, making sure not to splash it up over the sides of the pan. With your other hand, stir the mixture continuously to ensure that it's coming together in an even consistency. Let the sauce thicken. You might want to lower the stove's heat level a little bit so the sauce doesn't burn. The longer you let it cook, the thicker the sauce will be, so keep an eye on it. Stir the sauce continuously and let it cook until it reaches the consistency you like. The end product should have a smooth consistency. Make 1 cup medium white sauce as described above.  Chop the onion. Use yellow onions because they don't have as sharp a bite and they cook down well. Dice the onion into small pieces until you have about a teaspoon of onions. Prepare the mushrooms. If you're using canned mushrooms, drain them in a colander to remove all excess liquid. You don't want your sauce to be too watery, so drain them well. To prepare fresh mushrooms Remove the stems from the mushrooms by snapping them off with your hand. Dampen a paper towel with water. Wipe dirt off the mushroom heads one by one. You can rinse the mushrooms quickly in cold water, but don't let them soak because mushrooms absorb water very quickly. Melt the remaining 1 ½ tablespoons of butter or margarine. Put the butter in a separate skillet, and stir it over medium heat until it's melted. Add the mushrooms and chopped onion. Cook them over medium heat for a few minutes, until the onions change colors to a golden brown. You don't have to stir continuously, but stir every once in a while to make sure your mushrooms and onions aren't sticking to the pan. Add the mushrooms and onions to the white sauce to finish. Continue to heat the mixture over low heat for a minute or two, stirring occasionally. This will marry the flavors together and give you a cohesive sauce. Taste the sauce to see if it needs any more salt or pepper. Add salt and pepper while you're still warming the sauce. Adding seasoning after you've taken the sauce off the heat will result in a sharp, unpleasant saltiness. 
Melt 2 tablespoons butter or margarine. Add two tablespoons of flour, ½ teaspoon of salt, and black pepper to taste. Add 1 cup of milk. Let the sauce thicken. Make 1 cup medium white sauce as described above. Chop the onion. Prepare the mushrooms. Melt the remaining 1 ½ tablespoons of butter or margarine. Add the mushrooms and chopped onion. Add the mushrooms and onions to the white sauce to finish. 