Dill Rye bread
Grey Poupon Dijon Mustard (not the country style or another brand)
sweet pickle relish
avocado (in small chunks)
Muenster cheese, about 1/4" thick (creamy Havarti can be substituted)
Mayonnaise for frying (yes, MAYO. And I don't mean Miracle Whip - that's not MAYO)
Build the sandwich like any grilled cheese. A small amount of mustard on one inside face, relish on the other, cheese and avo in between. The mayo on the outside adds a slight flavor and a crisp crustiness. If you don't want to use mayo you can sub olive or coconut oil.
Extra mustard as a dipping sauce if you're into it.