Wash and dry the spring mix thoroughly. Peel (if desired) and thinly slice the cucumber into half-moons about 1/8 inch thick. Halve, pit, and slice the avocado into 1/4-inch slices. Toss the spring mix, cucumber, shredded carrot, green onion, and half the cilantro in a large bowl; set aside chilled.

Season tuna

Pat the tuna steaks dry with paper towels. Season both sides with kosher salt and freshly ground black pepper (about 1/2 tsp each side distributed from the measured amounts).

Coat tuna with sesame

Place the 3 tbsp mixed sesame seeds (from 'Coating & Pan') in a shallow dish. Press each seasoned tuna steak into the seeds, coating the top and sides well so seeds adhere; leave the bottom without seeds if you prefer a flat sear surface. Gently press to form an even crust.

Sear tuna

Warm a heavy skillet (cast iron recommended) over medium-high heat until very hot, about 2–3 minutes. Add 2 tbsp neutral oil and swirl. Place tuna, crusted-side down, into the pan. Sear for 45–60 seconds per side for rare center (adjust 90+ seconds per side for medium). If using butter, add 1 tsp unsalted butter and baste the top edge for 10 seconds. Remove tuna to a cutting board and let rest 2 minutes.

Slice tuna

Using a very sharp knife, slice the tuna across the grain into 1/4-inch thick slices. Arrange slices atop the prepared spring mix and vegetables. Drizzle the toasted sesame oil (1 tsp) over the tuna and salad for added aroma.

Sesame Cilantro Dressing

Make dressing in a blender or food processor: add 1 cup packed fresh cilantro, 3 tbsp light soy sauce, 2 tbsp rice vinegar, 2 tbsp fresh lime juice, 1 tbsp honey, 2 peeled garlic cloves, 1 tsp grated ginger, 3 tbsp neutral oil, 1 tsp toasted sesame oil, and 2 tbsp water. Blend until smooth. Taste and season with 1 tsp salt and 1/2 tsp freshly ground black pepper, adjusting honey or lime to balance sweetness/acidity. If too thick, add 1 tbsp water at a time to reach desired pourable consistency.

Transfer dressing to a jar and chill briefly if not using immediately. Shake or stir before dressing the salad.

Dress salad: just before serving, toss the spring mix and vegetables with about 3–4 tbsp dressing (reserve extra to serve at the table). Arrange tuna slices on top and spoon any remaining dressing lightly over the tuna and avocado.

Finishing & Plating

Garnish with remaining chopped cilantro and the extra 3 tbsp sesame seeds sprinkled over the plate. Serve immediately so the tuna remains warm inside and the greens are crisp.

Optional: serve with lime wedges on the side and extra dressing in a small pitcher.