Pat the cod fillets dry with paper towels, then season generously with salt and pepper. Zest the lemon directly onto the fish, then set aside the lemon for juice.
Preheat your skillet over medium heat until it feels warm but not smoking, about 3 minutes. Add the olive oil and let it shimmer.
Gently place the cod fillets skin-side down if they have skin. Let them cook undisturbed for about 3-4 minutes, watching as the edges turn golden and crispy, and the fish begins to release easily from the pan.
Carefully flip the fillets using tongs and cook for another 2-3 minutes until the fish is opaque and flakes easily with a fork. The edges should be beautifully golden brown.
Remove the cooked fish to a plate and set aside. Reduce heat to medium-low. In the same pan, add the minced garlic and cook for about 30 seconds until fragrant, stirring constantly to avoid burning.
Squeeze the lemon juice into the pan, scraping up any flavorful bits from the bottom. Let it bubble gently for a few seconds.
Add the cold butter pieces to the pan, whisking constantly until the sauce becomes silky and shiny. Taste and adjust with more lemon juice or salt if desired.
Return the fish to the pan briefly, spooning some sauce over the top to reheat and flavor it further, about 30 seconds.
Transfer the fish to a serving plate, spoon the lemon butter sauce over, and garnish with chopped parsley or dill. Serve immediately with extra lemon wedges if you like.
