Yes, I have started eating Sardines again. I have not found any in mustard (typical in the USA). But they come in almost every other marinade you can image. I particularly like the fillets of Sardine vs the full Sardine.
All the same a tin plus crackers and black pepper, make for an excellent lunch once or twice a week. They had some that were marinated in olive oil, hot pepper, olives, and pickles - they were awesome, I haven't seen them in quite some time. I'm doing the Italian marinade and the classic extra virgin olive oil for now. There is also a tomato, basil and olive version of the fillets that is pretty good with a splash of cayenne pepper or black pepper. There are also lemon "citron" marinade that are good, but they need quite a bit of spice so either hot sauce or pepper is added.