Stir in shrimp and heat through if using cooked shrimp (If using raw let cook until translucent-about 3 minutes).
Push shrimp aside and add flour to the liquid whisking with a fork --or, even better use a flour slurry.
Cook about a minute, until the sauce thick. Remove from heat.
Serve over cooked pasta with a bit of fresh parsley.
Note: to keep your sauce from getting lumpy it is best to create a flour slurry by mixing the flour with just enough water to make the mixture the texture of mashed potatoes. Whisk that into the sauce rather than simply adding flour in directly.