Preheat your oven to 400°F and line a small baking sheet with parchment paper.
Trim the woody ends off the asparagus, place them on the baking sheet, drizzle with 1 teaspoon of the olive oil, and season with a pinch of sea salt and pepper.
Roast the asparagus for 10-12 minutes until tender and slightly crisp at the tips.
While the asparagus roasts, prepare the lemon-herb sauce by whisking together the lemon juice, minced garlic, chopped parsley, capers, and 1 teaspoon of olive oil in a small bowl.
Pat the tuna steak dry with a paper towel and season both sides with the remaining sea salt and black pepper.
Heat the remaining teaspoon of olive oil in a cast-iron skillet over medium-high heat until shimmering.
Place the tuna in the hot skillet and sear for 1.5 to 2 minutes per side for medium-rare, or until your desired level of doneness is reached.
Remove the tuna from the pan and let it rest for 2 minutes before slicing.
Serve the sliced tuna over the warm quinoa with the roasted asparagus on the side, drizzling the lemon-herb sauce generously over the fish.