Preheat water to 122°F using a sous vide machine. (I use the Anova sous vide precision cooker.)
Prepare mackerel fillets by rinsing under cold water. Pat dry with paper towels. Season the fillets generously with salt and freshly ground black pepper on both sides.
Transfer the fish into a large sous vide bag in one single layer, ideally leaving a little room in between each fillet. Vacuum-seal the bag using a hand pump or a vacuum sealer.
Once the water bath is ready, lower the bag into the water, making sure that the fish is completely under the water. Use a heavy object to weigh it down if necessary. Set timer for 20 minutes.
Meanwhile, make the spicy cilantro lime dressing by combining all ingredients in a food processor or blender, blend on high speed for 30 seconds until thoroughly mixed.
When the timer goes off, remove the bag from the water bath and remove the mackerel fillets from the bag. Pat dry with paper towels.
Heat oil in a large nonstick pan or a cast-iron skillet over medium-high heat. Once the oil is hot, place the fish into the pan, skin side down, and sear for 30 seconds or until the skin is brown and crispy.
Dish and drizzle with spicy cilantro lime dressing. Enjoy!
If the spicy cilantro lime dressing isn’t your thing, simply squeeze some fresh lemon juice over the mackerel or garnish with lemon slices.
The spicy cilantro lime dressing recipe yields 1 cup. Keep the leftover dressing in an airtight container and refrigerate for up to one week.