My favorite once-a-year sandwich:
Grilled cheese and chili sandwich.
On the first day that the Farmer's Market vendors roast green chilis, I buy a bag of fresh roasted Anaheim or poblano chilis. I peel and dice one chili and mix it with minced garlic. I spread it between 2 slices of American cheese so that the gooey, melty cheese will trap the bits of chili/garlic goodness. I eat it with a chopped salad of avocado, cucumber, tomato, onion, sometimes fresh corn, or bell peppers, whatever I have, or bought at the market. No dressing, just a little salt and pepper.
My second favorite is a vegetarian sandwich that I only get during Lent. It probably defeats the purpose because I like this sandwich so much, it really isn't a sacrifice. A coffe shop in town that bakes their own bread sells it and it is all about the bread, the yummiest honey wheat rolls I've ever tasted. It has provolone, very thinly sliced tomatoes, cucumbers, bell pepper and onions, avocado, sprouts, mayo and dijon mustard. Mm-mm-mm!