In a small bowl, whisk together olive oil, vinegar, and Dijon; season with salt and pepper.
In a large bowl, combine tuna, lettuce, radishes, and avocado. Drizzle with dressing as desired.
Drizzle salad with vinaigrette, as desired, and toss to combine. Serve sprinkled with pepper.
This is such a great salad. It's super easy to prepare, but, it's much better with the fancy imported tuna in the glass jar (ortiz, I think) I got it at Whole Foods. We eat this at least once a week in the summertime.