Place baby potatoes in a pot of salted water, bring to a boil, and cook for 10-12 minutes until tender.
Pat the tuna steak dry with a paper towel and season both sides with sea salt and black pepper.
Heat avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Sear the tuna for 1.5 to 2 minutes per side for a perfect medium-rare center, then remove from the pan to rest.
In the same skillet, add the asparagus and cherry tomatoes, sautéing for 4 minutes until the vegetables are tender-crisp and blistered.
Wipe the skillet clean, melt the ghee over low heat, and whisk in the minced garlic, lemon juice, lemon zest, and chopped parsley.
Slice the tuna against the grain and serve alongside the potatoes and vegetables, drizzling the warm lemon-herb sauce over the top.