A lighter version of a creamy classic I sometimes substitute peas for the broccoli
Standard heavy cream uses high butterfat content to achieve viscosity naturally through emulsion, but reduced-fat iterations require a built-in starch matrix. Whisking all-purpose flour directly into cold skim milk and chicken broth before heating prevents dry pockets from forming, which ensures a smooth base when exposed to medium-low heat. Cook the liquid mixture until it coats the back of a spoon, signaling that the starch granules have fully gelatinized and swollen to trap the surrounding liquid.
Dairy solids in skim milk scorch easily because water evaporates quickly from low-fat milk products when heat exceeds 212 degrees Fahrenheit. Keep the saucepan at a gentle simmer and stir continuously with a flat-bottomed wooden spoon to scrape the bottom clean. Adding grated parmesan cheese only after the white sauce has thickened completely prevents the cheese proteins from tightening prematurely into rubbery clumps.
Boiling fettuccine requires a rolling boil in at least four quarts of salted water to prevent surface starch from making the strands gluey. Dropping frozen broccoli florets into the pot during the final three minutes of cooking utilizes residual heat efficiently without turning the vegetable gray and mushy. Swapping frozen peas for the broccoli alters the moisture release rate slightly, meaning you may need to thin the final sauce with two tablespoons of reserved pasta cooking water.