Prepare the vegetables: Heat the olive oil in a pot over medium heat. Add the onions and cook until golden, 3 to 4 minutes. Add the fennel and cook until slightly soft, 2 to 3 minutes. Stir in the sugar and season with kosher salt and pepper. Add the vinegar, garlic and 1 cup water; bring to a boil, stirring, then reduce the heat to medium low and simmer until the onions are tender, about 25 minutes.
Meanwhile, prepare the fish: Finely chop the pine nuts by hand. Mix the nuts, lemon zest, parsley, and sea salt and pepper to taste on a cutting board. Pat the swordfish dry and brush with olive oil on both sides. Press the fish into the nut mixture, turning to coat.
Heat a large nonstick skillet over medium-high heat. Add 2 tablespoons olive oil and swirl to coat. When the oil shimmers, lay the fish in the pan and cook, undisturbed, until golden on the bottom, 4 to 5 minutes. Flip and cook until just golden on the other side, 3 to 4 more minutes. Using tongs, lay each swordfish steak on its side to sear the edges, 1 to 2 minutes. Reduce the heat to medium low. Turn the fish so the lighter side is down and cook until just firm to the touch, 6 to 8 more minutes. Transfer to a cutting board.
While the fish rests, finish the vegetables: Add the tomatoes, oregano, capers and brine to the onions and cook until the tomatoes soften, 5 to 8 minutes. Season with kosher salt and pepper.
Slice the fish and divide among plates. Spoon the vegetables and juices onto the plates. Drizzle with the lemon juice and olive oil.