Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
In a large bowl, toss the broccoli florets with 1 tablespoon of olive oil, half of the sea salt, and half of the black pepper until evenly coated.
Spread the broccoli in a single layer on the baking sheet and roast for 15-20 minutes until the edges are golden and tender.
While the broccoli roasts, pat the tuna steaks dry with a paper towel and season both sides with the remaining salt and pepper.
Heat the remaining 0.5 tablespoon of olive oil in a cast-iron skillet over medium-high heat until shimmering.
Carefully place the tuna in the skillet and sear for 1-2 minutes per side for medium-rare, or until your desired level of doneness is reached.
In a small ramekin, whisk together the lemon juice, lemon zest, minced garlic, parsley, and dill.
Remove the tuna from the heat and immediately drizzle with the lemon-herb mixture, allowing the residual heat to release the aromatics.
Plate the tuna steaks alongside the roasted broccoli and serve while hot.