Peel the sweet potato, thinly slice, and blanch in water. Once the sum has been removed, drain the water. Slice the onion thinly as well.
In a pot, melt the butter and add the sweet potato and the onions. Pan-fry until softened, but be careful to make sure that they don't burn.
Add the water and the bouillon to the pot. Cover with a lid and cook for about 20 minutes. Simmer the vegetables until they are soft and tender. It's fine to heat over high.
Put the veggies in a blender and blend until smooth. Then, add then right back into the pot.
Cook on a low heat setting while adding the milk and mixing well. Keep adding until it reaches a consistency you prefer. Season with salt and white pepper, and it's done!
Story Behind this Recipe
Check out the carrot version at Recipe ID: 732350
The trick is not to burn the dish while cooking. You could use both potatoes and sweet potatoes for this. Sweet potatoes are smaller than regular potatoes, so I took the carrot and potato recipe and halved the ingredients.