1. Combine tuna with three tablespoons olive oil in a large mixing bowl. Sprinkle generously with sea salt and pepper, and toss until fully coated.
2. Set aside to marinate for a minimum of 30 minutes.
3. In the meantime, prepare salad ingredients and mix ingredients for dressing.
4. Heat more oil in a large pan and add tuna, spreading over surface of pan. Sear over high heat, turning to brown on all sides, for about five to six minutes or until desired doneness is reached. Set aside tuna in the pan. Place chopped leek and mushrooms into frying pan. Add additional oil and sauté until mushrooms begin to shrink and leeks soften and begin to brown. Pour vegetables over tuna and toss gently to combine.
5. In a large mixing bowl, combine all salad ingredients and mix in dressing. (You may not need all of the dressing; use as desired.) Top with tuna-mushroom mixture. Alternatively, divide salad among 10–12 small salad cups or bowls and top with the tuna-mushroom mixture.