Finely chop your cabbage and celery into strips. Grate carrots or use a peeler to turn them into long thin strips.
Whisk together mayonnaise, lemon juice, apple cider vinegar and mustard. Once ready, combine with cabbage, celery and carrots - but save two table spoons for topping your tacos later. Mix well.
Separate your grapefruits into segments and remove membranes. Slice up the avocado.
Once you're ready to assemble your tacos, put the coleslaw at the bottom and then layer grapefruit and avocado slices on top.