Preheat your oven to 400°F (200°C).
Trim the woody ends off the asparagus and chop the red bell pepper and zucchini into bite-sized pieces.
Place the vegetables on a large baking sheet and toss with 1 tablespoon of olive oil, garlic powder, and half of the sea salt and black pepper.
Roast the vegetables for 15-18 minutes until they are tender and show slight caramelized edges.
While the vegetables roast, pat the tuna steak dry with a paper towel and season both sides with the remaining salt and pepper.
Press the sesame seeds firmly onto the surface of the tuna steak to create a light crust.
Heat the remaining 0.5 tablespoon of olive oil in a cast-iron skillet over medium-high heat until the oil is shimmering.
Place the tuna in the hot skillet and sear for 1-2 minutes per side for a rare center, or longer depending on your preference.
Remove the tuna from the skillet and let it rest for 2 minutes before slicing into thin strips.
Drizzle the roasted vegetables with fresh lemon juice and serve immediately alongside the seared tuna.