Preheat your oven to 400°F (200°C).
Peel the sweet potato and dice it into 1/2-inch cubes.
On a parchment-lined baking sheet, toss the sweet potato cubes with 0.5 tablespoons of olive oil and a pinch of sea salt.
Roast the sweet potatoes for 20-25 minutes, tossing halfway through, until they are tender and golden brown.
While the potatoes roast, finely mince the fresh parsley, thyme, and garlic.
In a small bowl, combine the minced herbs, garlic, and lemon zest to create the crust mixture.
Pat the tuna steak dry with a paper towel and season both sides with the remaining sea salt and black pepper.
Press the herb mixture firmly onto both sides of the tuna steak to ensure it adheres.
Heat the remaining 0.5 tablespoons of olive oil in a non-stick skillet over medium-high heat.
Sear the tuna for 1.5 to 2 minutes per side for a perfect medium-rare center.
Slice the tuna against the grain and serve immediately alongside the roasted sweet potatoes.