Farmer's Market find: Negi

Today I am cooking one of my favorite "what to do with leftovers" dishes: Onion, leek, and potato soup. I like it because if you have extra mashed potatoes (and unless you're lucky enough to unload them on guests on the way out the door, then you do!), it's really simple. Slice onions, leeks, and garlic very thin. Sauté in oil until caramelized, pour in stock, bring to a boil, and then add mashed potatoes and stir. Let boil and/or simmer as you wish and that's it.
I didn't buy leeks earlier in the week, but this morning I was lucky to find negi, a long (long!) Japanese green onion-like vegetable. The white parts you see in the picture were about 10-12 inches long and there were an additional 12-18+ inches of the green parts. Carrying them away from the vendor's table, I felt like I was carrying spears. The guy who sold them to me said not to use the green parts, so I didn't.
It wasn't a leek, but close enough, and local from the farmer's market, so I had to give it a try in the soup. It's still simmering on the stove, and so far it looks and tastes good. I didn't have low-sodium stock so it's bordering on too salty. Other than that, I have high hopes.