The M.E. was asked a bunch of hypothetical questions. "Assume Nancy ate her last meal at 7pm then would that be consistent with a TOD of xx:xx." Assume Nancy vomited after coming home, would that be consistent with a TOD of xx:xx." The physical condition of Nancy's body, with the level of decomp, doesn't narrow the TOD by itself. But that onion segment was there and we know the approx time she left the party to walk across the street.
We aren't trying to prove Nancy died by 1am. So in looking at the food in her stomach, assuming she didn't vomit and all else was well, what's the maximum time it would take to digest ALL the food, including the onion? Certainly within 6 hrs. Oh but there was still an onion segment left in her stomach. Undigested.