Preheat the oven to 400°F (200°C).
Pour the 1 tablespoon olive oil into a small bowl. Use a little of it to lightly grease a baking dish and keep the rest for the fish.
Pat the 4 haddock fillets (about 6 ounces each) dry with paper towels.
Place the fillets in a single layer in the prepared baking dish, leaving a small space between each fillet.
Drizzle the remaining olive oil over the fillets, then evenly sprinkle 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper over them.
Sprinkle 1/4 cup grated Parmesan cheese evenly over the tops of the fillets.
Bake in the preheated oven for 15 minutes, or until the fish flakes easily with a fork and the cheese is slightly golden.
Serve hot, garnished with the lemon slices.