Preheat your oven to 400°F and line a small baking sheet with parchment paper.
Peel the sweet potato and cut it into 1/2-inch cubes.
Toss the sweet potato cubes with half of the olive oil, half of the sea salt, and the smoked paprika on the baking sheet.
Roast the sweet potatoes for 20-25 minutes, tossing halfway through, until they are tender and the edges are golden brown.
While the potatoes roast, bring a small pot of water to a boil. Carefully lower the egg into the water and simmer for 6.5 minutes for a jammy yolk.
Immediately transfer the egg to a bowl of ice water for 3 minutes, then gently peel and set aside.
Pat the tuna steak dry and season both sides with the remaining sea salt and black pepper.
Heat a cast-iron skillet over medium-high heat with the remaining olive oil. Once hot, sear the tuna for 1-2 minutes per side for a rare center.
Remove the tuna from the pan and let it rest for 2 minutes before slicing into 1/2-inch thick strips.
Place the baby arugula on a plate, top with the roasted sweet potatoes and sliced tuna, then finish with the halved egg and a drizzle of lemon juice.