1. Combine first 4 ingredients in a small saucepan over med-high heat; bring to a simmer. Remove from heat;cover and let stand 15 mins. Strain milk mixture through a sieve over a bowl;discard solids.
2. Wipe pan clean with paper towels. Melt butter in pan over med heat. Add flour to pan; cook 1 min, stirring constantly. Gradually add strained milk,stirring with a whisk until blended. Bring to a boil;cook 9 mins or until thickened,stirring constantly. Remove from heat,stir in salt and pepper.
Onion lends the sauce a faintly sweet note. For an even more aromatic bechamel, add a crushed garlic clove to the steeping milk.
For Mornay Sauce: Stir 1/2 cup (2 ozs) shredded Gruyere cheese into thickened Bechamel Sauce, stirring over med-low heat until smooth.
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