To make the pan sauce, add a tablespoon or two of olive oil and the sliced onion. Sprinkle a pinch of kosher salt over the onions and cook for about 3 minutes. Deglaze the pan with the port or wine, scraping the bottom of the pan to remove the fond, or brown bits, then add the Saskatoon berries. Let them cook down for a minute or so, then pour in the duck stock. Simmer until the sauce is reduced by about half, then add the knob of butter. Swirl the pan to incorporate the butter. Return the duck breasts back into the pan, turning to coat all sides with the glaze, then transfer to a serving plate and pour remaining pan sauce over the breasts.