Remove the tuna steak from the refrigerator 15 minutes before cooking to bring it closer to room temperature.
In a small bowl, whisk together 0.5 tablespoons of olive oil, lemon juice, lemon zest, minced garlic, chopped parsley, and chopped cilantro to create the herb sauce.
Pat the tuna steak completely dry with paper towels on all sides; this is crucial for achieving a proper crust.
Season both sides of the tuna steak evenly with sea salt and black pepper.
Heat a cast-iron skillet or heavy-bottomed pan over medium-high heat with the remaining 0.5 tablespoons of olive oil until the oil is shimmering.
While the pan heats, steam or lightly sauté the asparagus spears in a separate pan with a splash of water until tender-crisp and bright green.
Place the tuna in the hot skillet and sear for 1.5 to 2 minutes per side for a perfect rare to medium-rare center.
Transfer the tuna to a cutting board and let it rest for 2 minutes before slicing into 1/2-inch thick pieces.
Plate the tuna alongside the asparagus and spoon the lemon-herb sauce generously over the fish.