Boil potatoes in a saucepan of salted water until almost cooked through, drain.

Melt butter in the same saucepan over medium heat. Stir in flour and cook 1 minute while stirring until flour taste disappears. Gradually whisk in milk until mixture is smooth. Bring sauce to a boil. Add potatoes to sauce, reduce heat to low and simmer sauce 5 minutes. Add peas and cook 1 minute more. Season with salt and pepper and serve hot.