This recipe has become a weeknight standby for me. I thought 2 onions was a bit strong the first time I made this meal so the next time I used shallots and a different time leeks - both were great. I used sherry because I always have it on hand. My husband always says he doesn't like fish but he loves this recipe. I tend to use the swiss cheese because it's a bit less expensive than gruyere. I've made this with Cod and Talapia.