Melt butter in a saucepan over medium heat. Whisk in flour and cook for 1 minute.
Gradually whisk in chicken broth until smooth. Bring to a simmer.
Reduce heat to low and simmer for 5 minutes, or until slightly thickened. Stir in heavy cream, rosemary, salt, and pepper. Simmer for another 5 minutes.
Serve immediately over pasta, chicken, or fish.
Notes
For a richer flavor, use homemade chicken broth. You can also add a pinch of nutmeg or garlic powder.