Bechamel Sauce

I don't believe I've posted this before. So here it is: ( I don't make the full amount - I've always been facinated by science)

                                     Bechamel Sauce
                   (prep 30 mins total 30 mins makes 4 cups)
Ingredients: 

3 tablespoons  butter 
1/4 cup all-purpose flour
1/2 teaspoon coarse salt
4 cups whole milk
Directions:
1. In a medium saucepan, melt butter over medium. Add onion and cook until softened, about 6 minutes. Add flour and salt; cook, stirring frequently, until mixture is pale golden, has a slightly nutty aroma, and is the texture of cooked oatmeal, about 5 minutes.
2. Whisking constantly, pour in 2 cups milk; add 2 more cups milk and whisk until smooth. Cook mixture, stirring constantly along the bottom of pan, until boiling, about 7 minutes.
3. Reduce heat to low. Simmer gently, stirring occasionally, until sauce thickens and does not feel grainy when a very small amount is rubbed between two fingers, about 10 minutes.