Drain the sardines and gently break them into chunks in a medium-sized bowl.
Add the cubed avocado and mayonnaise to the bowl. Gently toss the ingredients together until they are combined, being careful not to mash the avocado completely.
Separate the endive leaves and arrange them on a serving platter.
Spoon the sardine and avocado mixture evenly into each endive leaf.
(Optional) Sprinkle with finely chopped red or green onion for added flavor.
Serve immediately and enjoy!
Notes
Leftovers: Refrigerate in an airtight container for up to three days.
No Endive: Use lettuce leaves, radicchio leaves, sweet potato toast, crackers, or bread instead.
Additional Toppings: Add green onions, chives, red onions, fresh or dried herbs, and/or dijon mustard for more flavor
No sardines: If you aren't a fan, try mixing them with salmon or use canned salmon.