canned sardines or mackerel are good, provided you choose well.
Good bread, some fish on it, and you have a nice little sandwich. It is by no mean an everest of fine taste and an orgasm of reactions. But it is quite good, even with that strong aroma and sent.
Regarding quality, I have no doubt they use the good ones at the polar star dorm.
Now, is it really possible to get something very good out of it, probably. But you really have to find ways to tone the sent and taste down.
Use hot boiled potatoes cut in 5 to 8 milimeters slices, oignons, shallot, red wine vinegar and canned sardines and you already start getting something real nice. Add some rucola salad (arugula in the States it seems, roquette in France) and it's really hard to stop eating that.
Works well with canned mackerel too.

Ideas are not that bad in the manga/show, there is some documentation. But there are mistakes and the anime delivery is subpar.