Place flour and milk in a blender and blend until well mixed and smooth.
Saute onion in oil on medium heat until tender.
Add shrimp and cook, stirring until translucent, about 2 - 3 minutes.
Pour liquid in and cook over medium heat stirring until thickened, making sure the shrimp is done, but not overcooked (a total cooking time of 3 to 5 minutes is usually enough for shrimp depending on the size.
Add , salt and pepper to taste.
Use over pasta or rice and add Parmesan cheese or flakes of red pepper if desired.
Note: cream sauces break or coagulate due to high heat, acids or salt. Fat in a sauce helps prevent this, but if you use skim milk as I do, keep the sauce from boiling if possible and add the salt at the end.