Rinse the quinoa under cold water and combine with 0.5 cup water in a small saucepan; bring to a boil, then reduce heat to low and simmer covered for 12-15 minutes until fluffy.
Pat the tuna steak dry with paper towels and season both sides evenly with sea salt, black pepper, and garlic powder.
Heat 0.5 tablespoon of olive oil in a cast-iron or heavy-bottomed skillet over medium-high heat until it begins to shimmer.
Place the tuna in the hot skillet and sear for 1-2 minutes per side for a rare-to-medium-rare center with a beautiful golden crust.
Remove the tuna from the pan and let it rest on a cutting board; in the same skillet, add the asparagus spears and sauté for 3-4 minutes until tender-crisp.
In a small bowl, whisk together the remaining 0.5 tablespoon of olive oil, lemon juice, lemon zest, and finely chopped fresh parsley.
Slice the tuna steak against the grain and serve it over the cooked quinoa and asparagus, finishing with the zesty lemon herb drizzle.