Melt the butter in a large soup pot and saute the onion, leeks, and potato for about 5 minutes, stirring often.
Add the stock to the pan and bring up to a simmer. Cover, turn down the heat and simmer for about 15 minutes, or until everything is tender. Add the peas to the pot just long enough to defrost them.
Working in 2 batches, process the soup until your desired consistency. It can be smooth, or chunky. Season to taste with salt and pepper.
Serve piping hot with a drizzle of sour cream and a sprinkle of chives.