Preheat your oven to 400°F and line a small baking sheet with parchment paper.
Toss the asparagus spears with 0.5 tbsp of olive oil and a pinch of salt and pepper; roast for 10-12 minutes until tender.
While the asparagus roasts, pat the tuna steak completely dry with paper towels to ensure a high-quality sear.
Season both sides of the tuna steak evenly with the garlic powder and the remaining sea salt and black pepper.
In a small mixing bowl, whisk together the lemon juice, lemon zest, chopped parsley, and 0.5 tbsp of olive oil to create the herb sauce.
Heat the final 0.5 tbsp of olive oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Place the tuna in the hot skillet and sear for 90 seconds to 2 minutes per side for a perfect rare-to-medium-rare finish.
Remove the tuna from the pan and let it rest for 2 minutes before slicing it against the grain into thick strips.
Plate the sliced tuna alongside the roasted asparagus and sliced avocado, then spoon the lemon-herb sauce over the fish.