Preheat the oven to 375°F (190°C).
Season the filets with salt and pepper on both sides.
Add the oil to a large oven-safe skillet over medium heat.
Once hot, add the filets, skin side down, and cook for 3 minutes, until the skin is crispy.
Flip the filets carefully and cook for an additional 2 minutes.
Remove the filets from the skillet and set them aside on a plate.
In the same skillet, reduce the heat to medium and add the butter.
Once melted, stir in the minced garlic and cook for 1 minute until fragrant.
Add the lemon juice to the skillet and stir to combine.
Return the cod filets to the skillet, spooning the lemon butter sauce over the tops.
Transfer the skillet to the preheated oven and bake for 5-7 minutes, until the fish registers 130° F(50°C) on a meat thermometer.
Sprinkle the lemon zest and parsley over the top.
Serve garnished with lemon slices and a spoonful of lemon butter sauce.