Place fingerling potatoes in a pot of cold salted water, bring to a boil, and cook until tender, approximately 10-12 minutes.
Carefully add the egg to the same boiling water for the last 7 minutes of potato cooking time to achieve a jammy yolk, then transfer to an ice bath immediately.
Blanch the green beans in a separate pot of boiling water for 2 minutes until bright green and crisp-tender, then shock in ice water to stop the cooking.
In a small bowl, whisk together the extra virgin olive oil, red wine vinegar, Dijon mustard, and half of the sea salt and black pepper to create the vinaigrette.
Pat the tuna steak dry with a paper towel and season all sides with the remaining sea salt and black pepper.
Heat avocado oil in a cast-iron skillet over high heat until shimmering, then sear the tuna for 1-2 minutes per side for a rare center.
Slice the seared tuna into thin strips, peel and halve the egg, and slice the cooked potatoes into rounds.
Arrange the mixed greens on a large plate and top with the tuna, egg halves, potatoes, green beans, and Kalamata olives.
Drizzle the prepared Dijon vinaigrette over the entire salad and serve immediately.