Preheat your oven to 400°F (200°C). Toss the asparagus spears and cherry tomatoes with half of the avocado oil, a pinch of sea salt, and black pepper on a baking sheet.
Roast the vegetables for 10-12 minutes until the asparagus is tender-crisp and the tomatoes have just begun to burst.
While the vegetables roast, prepare the herb sauce by whisking together the extra virgin olive oil, fresh lemon juice, finely minced garlic, and chopped fresh parsley in a small bowl.
Pat the tuna steak dry with paper towels and season both sides with the remaining sea salt and black pepper.
Heat a cast-iron skillet over medium-high heat with the remaining avocado oil until the pan is shimmering and very hot.
Sear the tuna for 1.5 to 2 minutes per side for a rare center, or longer if you prefer it more well-done.
Remove the tuna from the pan and let it rest for 2 minutes before slicing against the grain.
Plate the sliced tuna alongside the roasted vegetables and spoon the lemon-herb sauce generously over the fish.