Bring the olive oil to medium heat in a wide-bottomed pot. Add the onions, potatoes, and carrots and sauté until the onion becomes translucent, about five minutes.
Add the vegetable stock and bring to a rapid boil.
Add the beans, the buckwheat and fennel seeds. Put the lid on and simmer at medium high for fifteen minutes, until both the potatoes are done and the buckwheat is soft but still has some bite – much like al dente spaghetti.
Stir in the tahini. It will gobble up liquid. So if the stew becomes too dry, add a bit of hot water.
Taste and add salt if needed.
Add the frozen corn. As soon as the stew returns to a simmer, one minute or less, take it off the heat and serve.