The actual reason this is strained is to remove the solid onion and bay leaf.
Béchamel, one of the "mother" sauces in classic French cuisine, is versatile: It's used in dishes such as lasagna, macaroni and cheese, and moussaka.
How to Make It
Step 1
Combine first 4 ingredients in a small saucepan over medium-high heat; bring to a simmer. Remove from heat; cover and let stand 15 minutes. Strain milk mixture through a sieve over a bowl; discard solids.
Step 2
Wipe pan clean with paper towels. Melt butter in pan over medium heat. Add flour to pan; cook 1 minute, stirring constantly. Gradually add strained milk, stirring with a whisk until blended. Bring to a boil; cook 9 minutes or until thickened, stirring constantly. Remove from heat; stir in salt and pepper.