Pat the mackerel very dry, especially the skin. Season the flesh with salt, black pepper, garlic powder, and lemon zest.
Heat a large skillet over medium-high heat and add the olive oil. Let the pan heat until the oil moves easily across the surface.
Place the fillets skin-side down. Press gently with a spatula for the first 20 to 30 seconds, then cook for 4 to 6 minutes until the skin is crisp and most of the flesh looks opaque from the sides.
Turn the fillets carefully and cook the flesh side for about 1 to 2 minutes, just until the center is cooked.
Transfer to plates, squeeze over fresh lemon juice, sprinkle with parsley, and serve with lemon wedges.
Notes
Dry skin and steady medium-high heat are the keys to crisping. Press the fillets gently for the first 20 to 30 seconds to prevent curling.