Prepare the Beets: Peel and roughly chop the beets into 1-inch cubes. Boil in water until tender, then set aside to cool.
Chop and Mix: Dice the celery, red pepper, green onion, and avocado. In a bowl, mix these with the drained tuna until well combined and the texture is to your liking.
Combine and Season: Add the cooled beets to your tuna mixture. Drizzle with mayonnaise or olive oil, then gently toss everything together to coat evenly.
Garnish and Serve: Top your salad with extra slices of green onion and avocado. Add a dash of pepper to taste and serve either chilled or at room temperature.