In a deep casserole or pan, put water and some salt to boil, add the celery (and leeks, if you have decided to use them in your recipe), and boil for about 5 minutes. Drain and set aside.
In the meantime, cut the pork into big stewing chunks and finely chop the onion.
Add olive oil to a pan, when it's hot, add the finely chopped onion as well as the pork pieces and let it cook until everything gets a nice golden brown shade.
Add some water (about 2 to 3 cups, depending on the sice of your pork pieces) and let the meat cook for about 45 minutes with the lid on.
After that time has passed, the pork will be quite tender. It's time to incorporate the celery (or celery and leek), as well as salt and pepper.
Cover again and let it cook in a very low flame. Avoid stirring. Instead, it's a better idea to just shake the casserole a bit so the meat does not stick on the bottom. If more water is necessary, then add it to the casserole, but do not overdo it.
The dish is ready once the water has been absorbed and the meat is tender. Add the lemon juice, let it sit for a couple of minutes, and serve hot.