In a medium saucepan, add the potatoes and carrots and cover with water. Bring to a boil and then reduce to a simmer. Cook until the veggies are soft enough to poke a fork into. Remove the pot from the heat and let cool a bit.
Using a slotted spoon, add the potatoes and carrots and chunk of onion to the blender. Add the water from the cooking pot.
Add all the other ingredients and pulse to start mixing. Blend on high power until you have a smooth sauce.
Pour the sauce into a saucepan and heat on medium heat, stirring often.
Bring to a slow boil and stir constantly as it thickens. It will get to the consistency of pudding.
Remove from heat and serve or store in a glass container for easy reheating in the microwave. Use cold as a spread on toast or as a filling for a grilled cheese sandwich. It thickens to a firm, almost sliceable cheese when chilled.