Lettuce Bun: Take the head of lettuce and lay it on a cutting board; use a large knife to cut off two slices from the lettuce to create buns (see photos in post for an example).
Mayo: Take one of the lettuce buns and add as much mayo as desired.
Add Fillings: Top with red onion, deli meat, tomato, and guacamole (if desired).
Serve: Place the other lettuce bun on top and enjoy!
Notes
Whole30/Paleo: check ingredients of the mayo to ensure it is compliant or make a Whole30 homemade mayo. Also, check the ingredients of the deli meat.
Meal Prep: You could definitely make these sandwiches the day before work and store in the refrigerator overnight in an airtight container. If you want to store for longer, then I wouldn't add the mayo or guacamole until ready to eat, so the lettuce doesn't get soggy.