The crispness of the romaine lettuce and radishes contrasts nicely with the smooth richness of the ripe avocado slices and the saltiness of the tuna.
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.