To make the steamed, puréed sweet potatoes, peel and cut 2 large or 4 medium sweet potatoes into large chunks. Add to a steamer basket set over simmering water. Cover and steam until potatoes are soft enough to mash with a fork. Transfer the hot potatoes to a blender or into a bowl if you have an immersion blender. Add a little of the water, about 1/4 c. from the steam pot to the potatoes. Blend until smooth. I like to make 4 or 5 potatoes at a time, purée, let cool, and then divide into individual 1 c. portions in freezer bags and freeze for future use.
Over medium low heat add butter to a sauce pan. Keep it on the heat until it begins to brown, careful not to burn.
Add flour, whisking continuously for 3-4 minuets.
Turn the heat up to medium, add milk and whisk until combined and smooth, not more than 6 minutes. Whisk occasionally carful not to burn the bottom and not to boil.
Turn heat down to low. Add cheese, salt and pepper. Whisk until cheese is melted.
Add puréed sweet potato, whisk to combine.
Sauce will thicken as it cools. If it becomes too thick add more milk 1/4 cup at a time over low heat until it reaches a desired consistency.