Add the sweet potato, milk, garlic, salt, mustard and nutmeg
Blend until smooth and set aside
Make the roux
Heat 1/2 cup of neutral flavored oil on low heat. When it's hot add 3.5 ounces of flour. Whisk to blend the flour and oil together and cook on low for two minutes.
Raise the burner to medium high and add the contents of your blender. Bring to a simmer.
When it starts to bubble reduce heat to medium and cook for five minutes. Keep stirring to break up the sweet potato. After five minutes add the lemon and whisk in.
Cook Your Pasta
Bring a pot of water to boil
Add your pasta and cook according to the manufacturer directions.
When the pasta is almost done add half a cup of your pasta water to the sauce and whisk it in