I have no idea what vegetarians do ... perhaps one will show up and tell us.
As for liking fish ... that would be me.
The Friday Abstinence is still a pennance ... for one thing, it’s a constraint ... for another, there are plenty of ‘penitential’ fish dishes: tuna casserole comes to mind. If even that is insufficiently penitential for someone, nobody says you have to eat fish.
All of us engaged in the same penitential act also serves as a sign of unity.
Finally, I note that someone already has the “Irrelevant Scripture Generator” fired up. I recommend ignoring it. YMMV.
I highly recommend a Chickpea Dish! For me, penance is making the meals my family wants (and cleaning up after them) instead of what I want (toast).