In a medium pot, cook the peas in a bit of water. About 5 minutes (you don't want mushy peas!)
Drain the peas and put the pot back on the burner.
Melt the butter and add the flour, stirring to make a roux.
Add the milk and whisk to blend. Bring to a soft boil.
Add the cheese and it should thicken very quickly.
Stir in the peas and S&P to taste.