In a small skillet, melt butter; add onion and saute until golden.
Add flour to make a roux and cook 2 to 3 minutes.
In a medium saucepan, bring milk to a boil.
Pour a small amount of hot milk into roux, and stir until blended.
Add roux to remaining milk, whisking constantly.
Return to heat, add peanut butter, and continue to whisk until ingredients are heated through.