In a medium saucepan, melt the butter over medium heat. Then, add the flour and stir continuously for about 2 minutes until the mixture becomes smooth.
Gently whisk in the milk, stirring continuously as the sauce thickens for about 5 to 7 minutes. Be careful not to let it boil.
Stir the drained tuna and frozen peas into the sauce. Season with salt and pepper, and warm for an additional 2 to 3 minutes until heated through. Serve immediately over toasted bread if desired. Sprinkle with parsley for garnish.