Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Halve the baby potatoes and toss them on the baking sheet with 1 teaspoon of avocado oil and a pinch of sea salt.
Roast the potatoes for 15 minutes, then move them to one side of the tray and add the asparagus spears.
Drizzle the asparagus with another small drop of oil and a pinch of salt, then roast everything for an additional 10 minutes until the potatoes are golden and the asparagus is tender.
While the vegetables roast, whisk together the extra virgin olive oil, lemon juice, lemon zest, Dijon mustard, chopped parsley, and chopped chives in a small bowl to create the herb sauce.
Pat the tuna steak dry with a paper towel and season both sides generously with the remaining sea salt and black pepper.
Heat a heavy cast-iron skillet over high heat with the remaining 1 teaspoon of avocado oil until it begins to shimmer.
Place the tuna in the hot skillet and sear for exactly 1 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 into thick strips against the grain.
Plate the sliced tuna alongside the roasted potatoes and asparagus, then spoon the lemon-herb sauce over the fish and serve immediately.