Preheat your oven to 400°F and line a baking sheet with parchment paper for easy cleanup.
Trim the woody ends off the asparagus and place the spears on the baking sheet, tossing them with 0.5 tbsp of olive oil, minced garlic, sea salt, and black pepper.
Roast the asparagus in the oven for 10-12 minutes until they are tender-crisp and slightly charred.
While the vegetables roast, whisk together the Greek yogurt, finely chopped fresh dill, lemon zest, and lemon juice in a small bowl until the sauce is smooth.
Pat the ahi tuna steak completely dry with paper towels and season both sides generously with sea salt and black pepper.
Heat the remaining 0.5 tbsp of olive oil in a cast-iron or heavy-bottomed skillet over medium-high heat until the oil is shimmering.
Place the tuna in the hot skillet and sear for 90 seconds per side to achieve a golden crust with a perfect rare center.
Remove the tuna from the pan, let it rest for a minute, then slice it against the grain.
Plate the roasted asparagus and quinoa, top with the sliced tuna, and finish with a dollop of the creamy lemon-dill sauce.