Preheat the oven to 425°F (220°C).
Peel the kohlrabi and cut it into small cubes. In a bowl, toss the kohlrabi with half the olive oil, minced garlic, a squeeze of lemon juice, lemon zest (from the half lemon), and a pinch of salt and pepper.
Spread the kohlrabi evenly on a baking sheet and roast in the oven for about 20 minutes, or until tender and slightly caramelized, stirring halfway through.
While the kohlrabi roasts, pat the cod fillet dry with a paper towel. Season both sides lightly with sea salt and black pepper.
Heat the remaining olive oil in a non-stick skillet over medium-high heat. Once hot, place the cod fillet in the pan and sear for about 3-4 minutes per side until the exterior is crispy and the fish is opaque in the center.
Once cooked, plate the cod alongside a generous serving of the roasted lemon-garlic kohlrabi.
Finish the dish with an extra drizzle of lemon juice if desired, and serve warm.