After 45 minutes, add the chopped selino (celery or celeriac) and stir and cook for another 30 minutes. Add salt and white pepper to taste. If it looks like the broth has reduced to much, add a little boiling water. After the celery has cooked, remove the pot from the heat and make the avgolemono sauce.
Make the avgolemono sauce: In a seperate bowl, add the lemon juice and combine with the cornflour. In another seperate bowl, beat the egg and then slowly whisk in the the lemon juice mixture until frothy. Next, slowly add the broth from the pot whisking continuously (about 2 tea cups worth). Finally, add the sauce back into the pot covering the pork and celery. Return to a very very low heat and swirl the sauce by shaking the pot from the handles. Do not stir or allow the sauce to boil. The sauce will slowly thicken. Remove from the heat and then serve a slice or two of really good sourdough bread.
If you cannot find Greek selino (which is like a herb) substitute with ordinary celery. Peel and trim the stalks, removing the strings and slice into 3 inch pieces. In winter, you can also use celeriac. Peel and chop into bite size pieces.