Step 1
To a large pot over medium-high heat, add the pancetta and cook, stirring occasionally, until the fat renders, about 12 minutes. (If the pancetta begins to brown too fast, turn the heat to medium-low.) Using a slotted spoon, transfer the pancetta to a paper towel-lined plate and set aside to drain.
Step 2
Return the pot to medium-high heat and add the oil. When it’s hot and shimmering, add the onion and cook, stirring occasionally, until soft and light brown, about 10 minutes. Add the celery, tomatoes, and ¼ cup of water and season to taste with salt and black pepper. Cover the pot and cook, stirring occasionally, until the celery is very tender, about 1½ hours.