Melt the ghee in a soup pot and, when it's hot, add the onion, asafetida, rosemary and thyme. Give a stir and while they're cooking over medium heat, peel the sweet potatoes with a vegetable peeler, slice them into rounds and add them directly to the pot as you work. Continue to cook, stirring occasionally, until you see some coloring on the sweet potatoes. They will caramelize as they cook, which not only lends flavor but tempers their sweetness, so give them a good 15 or 20 minutes instead of the usual 5 or 7. Add the salt and water; bring to a boil, then simmer, covered, for 15 minutes. Let cool slightly, then puree, leaving a little texture if you wish or making it super smooth. Taste and add more salt if need be, but it should be fine.
FINISHING THE SOUP
You have options here. For sure, season each bowl with plenty of cracked pepper. Because this is a smooth-textured soup you might want to finish it with some tiny croutons, browned in ghee with minced rosemary until crisp, then added at the last minute.
Stirring a spoonful of sour cream or creme fraiche into each bowl of soup tempers the sweetness. I do that, then drizzle a few drops of tamari over each serving.
Chopped parsley always offers a clean and bracing counterpoint to sweetness.
A few drops of good balsamic vinegar would also be good. Smoked sea salt, as well.
Make a combo garnish: pepper, croutons, sour cream and soy sauce or smoked salt.