Preheat your oven to 400°F. Line a baking sheet with parchment paper.
Trim the tough ends from the asparagus and toss them lightly with half the olive oil, a pinch of salt, and black pepper. Arrange them on the baking sheet.
Place the asparagus in the oven and roast for 12-15 minutes until tender and slightly crisp.
While the asparagus roasts, prepare the cauliflower mash by steaming the cauliflower florets until very tender, about 8-10 minutes.
Once steamed, transfer the cauliflower to a food processor. Add the unsalted butter, a pinch of salt and pepper, and blend until smooth. Adjust seasoning as needed.
Meanwhile, pat the tuna steak dry with a paper towel. Season both sides with a pinch of salt and pepper.
Heat the remaining olive oil in a skillet over medium-high heat until just smoking. Sear the tuna steak for about 2-3 minutes on each side for medium-rare, or adjust to your desired doneness.
Plate the seared tuna alongside a serving of roasted asparagus and a generous scoop of cauliflower mash. Serve immediately.