Melt butter in saucepan large enough to hold all ingredients. Add flour and stir together; cook 1–2 minutes. Remove pan from heat and add milk; stir until smooth, return to heat, and bring to boil until thickened. Season with salt and pepper and add cooked carrots. Stir together just long enough to heat through. If sauce is thicker than desired, some carrot cooking water or can liquid may be added to adjust.