Melt butter in a medium saucepan over medium heat.

Whisk in flour and cook for 2 minutes to create a roux, ensuring it does not brown.

Gradually whisk in the whole milk until the mixture is smooth and lump-free.

Simmer the sauce for 3 minutes or until thickened enough to coat the back of a spoon.

Stir in salt, pepper, and onion powder.

Fold in the peas and cook for 5 minutes until heated through.

Serve warm, garnished with fresh parsley.