I made this soup yesterday. It's kind of like chowder and super tasty, so I thought I'd write the recipe down to prevent my forgetting it. It's easiest if the potatoes are left over from something else, but there's nothing wrong with cooking a potato for the soup, they don't need to be cold or anything.
1 already cooked sweet potato, skinned and mashed
1 stalk of celery, chopped
1/3-1/2 medium red onion, chopped (sweet onions would probably also work but I like red ones)
some butter to fry the onions in
1/2 cup frozen or canned corn
1 cup (or so) milk
salt to taste
black pepper to taste (I use maybe 1/2 a teaspoon)
You can add more vegetables as you have them. This soup began as a kind of leftover mashup, and this is what I had. I can verify that it's good this way, though.
First, fry the onion for a minute or two in a pot, then add the celery and fry for another minute or so, so the onion is more than halfway translucent and the celery is still crunchy. Remove from heat, and add the potatoes and corn. I never manage to mash the potatoes properly, so at this point I make sure there aren't any large chunks left in the potatoes by poking them around with a spoon. Add milk and enough water to make the soup the consistency you want it, which will depend on the size of the potato. Last time I made this I had a rather small potato and it took maybe a cup of water, although I can't be sure because I just stuck the pan under the faucet and filled it until it looked right. If you don't use enough water, the potato will suck it all up and instead of soup you'll have mash.
Return the pot to heat and warm it until just before boiling. Add spices to the point where it's a little salty and slightly peppery. Before you add the salt this soup will be pretty sweet, but it takes a surprisingly small amount to make it taste like chowder so don't get too enthusiastic. When it comes to the pepper, though, enthusiasm isn't a bad thing; I always end up adding more in the end. Garlic powder is optional.
This recipe makes approximately two servings, but of course you could make a huge pot of it if you happen to have several left over sweet potatoes.