In a small pan, boil peas while you prepare the white sauce. Boil until peas begin to dimple.
In a separate larger pan, melt butter.
Add onion and cook until onion starts to become translucent (2-3 minutes).
Add in flour and blend with butter.
Slowly add milk and continue stirring to allow milk to blend evenly into flour mixture and reduce lumps.
Add salt and pepper, as desired.
Simmer sauce until it thickens (about 10-15 minutes) while stirring frequently.
By this time the peas should be starting to dimple. Drain peas and pour them into the white sauce.
Continue to cook the creamed peas on low for about 5 minutes.
Serve over smashed boiled potatoes.