Step 1Melt the butter in a large soup pot or Dutch oven over medium-high heat. Add the onions and cook, stirring, until golden brown but still soft, about 5 to 8 minutes.
Step 2Add the stock, potato and salt and pepper to taste. (Grandma used water instead of stock, but I find stock adds a nice meaty flavor.) Lower the heat to medium, cover and simmer until the potato is just tender, about 15 to 20 minutes.
Step 3Uncover. Press the rivel batter through a colander with 1/4-inch holes into the soup (or use a ricer or spaetzle machine). Boil gently, stirring once or twice, until the rivels rise to the surface and are tender, about 5 to 10 minutes.