Rinse the celery stalks and leaves and pat dry. Peel the stalks (I highly recommend not skipping this step; you'll end up with stringy celery otherwise).
Cut the celery into 1 1/2-inch pieces and roughly chop the leaves. Cut any thicker stalks of celery lengthwise. Set aside.
Grate or finely chop the small onion. Finely chop the leek. Set them both aside together.
Trim excess fat from the pork butt and cut into 1 1/2-inch cubes. Season with salt and pepper.
Preheat olive oil in a 6-quart pot or dutch oven over medium heat. You want the oil to get hot, but not smoking. Add the pork and bones in batches and let brown, undisturbed, for 3 minutes per side. Set aside in a medium bowl.
Add onions and leeks to the dutch oven and let cook five to seven minutes, until softened. Stir occasionally.
Add the pork back to the pot and mix with the onions and leeks. Add enough water to barely cover the meat. Let come to a boil.
Reduce to a simmer and add in the celery. Cover with a lid, leaving a slight opening between the lid and dutch oven.
Cook for 1 1/2 - 2 hours, stirring occasionally. The pork will be tender and the celery will be soft. Remove from heat.
Immediately ladle out 3/4 - 1 cup of broth into a heatproof measuring cup.
Crack the egg into a small or medium sized bowl and squeeze in the juice of one lemon. Whisk until frothy. Continue to whisk and very slowly add in the hot broth to temper the egg.*
Add the frothy broth back to the pot and stir it in. Let rest for five minutes to thicken.
Taste and adjust seasoning by adding more salt and pepper if necessary. Remove neck bones if you'd like.*
Serve with crusty bread and enjoy!