Wipe the pork with damp paper towels, then cut into 1-1/2 inch cubes (the fat and skin may be left on during the cooking and removed later)
Melt 2 tablespoons of the butter in a heavy pan or Dutch oven
Add the onion and cook until soft and transparent, then add the pork and cook, stirring, over medium heat until the raw meat color disappears
Season with salt and pepper, add hot water to cover, then cover and simmer gently (or bake in a 325 F oven) for 30 to 35 minutes, or until almost tender
(The timing is important because the celery is to be added and cooked with the pork only until both are tender but not overcooked
)
Meanwhile, prepare the celery
Wash the stalks and scrape the heavy ones slightly
Cut each stalk once lengthwise (if large) and then across into 1-1/2 inch slices
(Use the leaves as well, if desired, but a few might be saved for a garnish or an accompanying salad
) Add the celery to pork and continue simmering 25 minutes until both are tender
Using a slotted spoon, remove the pork and celery and place in a serving dish, first removing and discarding the fat from the meat
Keep warm
Skim the fat from the cooking liquid, then add water or boil down rapidly to make to make 1-1/2 cups
Keep hot while you prepare the avgolemono
To prepare the avgolemono, heat the remaining 2 tablespoons butter in a pan
Stir in the flour, and after cooking over low heat for 1 minute, gradually add 2 cups of the hot cooking liquid from the meat
Stir until the sauce comes to a boil
Meanwhile, in a small bowl, beat the two remaining egg yolks and add the lemon juice, droplet by droplet, beating all the while
Beat a little of the thickened cooking liquid into the yolk mixture, then add the yolks to the pan of hot liquid
Mix well and cook over low heat until thickened
Pour the hot sauce over the pork and celery, garnish with parsley or celery leaves and serve warm