Given the task definition and input, reply with output. In this task, you will be presented with the directions of a recipe separated by "," and have to fill in the "___" which is a step that is missing from the recipe.

Melt the butter in a large pan.,Add the flour and stir over very low heat for a few minutes, until well blended.,Pour in a ladle of stock, beating vigorously to keep lumps from forming.,Add the rest of the stock gradually, stirring constantly.,Season to taste with salt and pepper, and bring to the boil slowly, stirring often.,Add the walnuts and fenugreek and simmer, covered, for 15 minutes, until the soup thickens a little and has lost any floury taste.,Just before serving, pour the yogurt into the simmering soup, beating vigorously.,Leave over low heat until it comes to just below boiling point, but do not allow the soup to boil, or it will curdle.,Add a little salt and pepper, taking into account the saltiness of the stock.,______

Serve immediately.