Pat the tuna steak completely dry with a paper towel and season both sides evenly with the sea salt and black pepper.
Heat half of the olive oil in a cast-iron skillet over medium-high heat until the oil is shimmering and just starting to smoke.
Place the tuna in the hot skillet and sear for 1-2 minutes per side for a rare to medium-rare center, then transfer to a plate to rest.
In the same skillet, add the remaining olive oil and sauté the asparagus spears for 4-5 minutes until they are vibrant green and tender-crisp.
While the asparagus cooks, whisk together the lemon juice, lemon zest, minced garlic, chopped parsley, chives, and capers in a small bowl.
Slice the rested tuna against the grain into thick, uniform strips.
Arrange the sliced tuna over the warm quinoa and sautéed asparagus, then drizzle the fresh lemon-herb sauce over the fish and vegetables.