what did the narrator mean by once we even ate off the plates

The girl meant that once they had eaten off the antique dinnerware. There was a pewter plate which seems to have been a precious heirloom and the narrator had always fancied an apple on that pewter plate. But the girl told the narrator that they used it for everything and once they had also eaten off it. The girl or Mrs Dorling would not have understood the sentimental value attached to these precious things of the narrator's family, it wasn't special to them.