Toasted whole grain bread, or a ciabatta roll if I have the money for it when shopping. Mayo and pepper to taste on each slice. Spicy brown mustard on one slice (or both, if you’re feeling saucy). Turkey/ham/moo meat as desired. Spinach on top, with a salted tomato slice on occasion.
This has been my go-to for years, and I can eat it just about every day without getting sick of it.
“Fundamental” is awfully subjective, though, isn’t it?