Meanwhile, in a large skillet, heat the olive oil over medium and add the dry Grünkern. Toast it, stirring, for 2-3 minutes. Careful not to let it burn.
As soon as the cabbage is tender, stir in the vinegar, and add the toasted Grünkern and the broth. Increase heat, and bring the mixture to a light simmer and cook for 30 to 35 minutes, until the Grünkern is tender. If needed, add another 1/2 cup (125 ml) broth or water to the soup. Remove the thyme sprig. Stir in lemon juice. Taste and season with more salt and pepper, to taste. Feel free to add a splash more red vinegar, if desired.