Preferably something that has little to no preparation required.
Fast as in the non-eating time since last meal or fast as in quick? I might have missed the original question… If it’s the latter, there’s plenty of quick, filling meal items in the morning. When I was serious about weight lifting, I would drink a protein shake 1-3 times a day to supplement muscle-building.
As for the former, intermittent fast-ers are adamant that anything other than water, plain coffee/tea will break their fast and provide less benefits.
I don’t know, but wonder if there is a scientific basis for this. If you’re avoiding eating something that’s going to fill you up (ie protein), my reaction is kind of “why bother”…