Heat butter in saucepan over medium high heat, add onions and saute until sl browned, but not too dark, 6-8 minutes.
Bring potatoes, stock and 1/2 tsp salt to a sismmer in a large pot. Cook until potatoes are tender, ~12 minutes.
Transfer potatoes to a food processor, leaving liquid in the pot. Pulse a few times, enough to reduce potatoes to small bite-sized bits, but not enough to puree. Do not overprocess.
Return potatoes to the soup pot, add the sauteed onions, Parm, tamari and milk, season to taste with S&P, return to simmer.
Add sour cream before serving.