Preheat a grill pan over medium heat.
Brush the 2 mackerel fillets (8 oz / 225 g) with 1. 0 tbsp. (0. 5 oz / 15 g) olive oil, and sprinkle lightly with salt and black pepper.
Place the fish onto the preheated pan and cook on both sides for 6 to 8 minutes until fully cooked and slightly charred, then set aside.
Bring 1. 5 cups (12. 0 oz / 340 g) of salted water to a rolling boil in a small pot over medium heat. Gently drop in the 2 large eggs (3. 5 oz / 100 g) and boil for exactly 10 minutes.
Once done, run the eggs under cold water to cool, peel off the shells, chop them into small pieces, and transfer them to a salad bowl.
Gently flake the cooled mackerel fillets into the salad bowl with the chopped eggs.
Add the diced 1 medium tomato (4. 3 oz / 122 g) and 2. 0 tbsp. (1. 0 oz / 28 g) mayonnaise to the bowl.
Mix everything evenly with a spoon until the ingredients are well combined.
Separate the 0. 5 head green lettuce (5. 3 oz / 150 g) into individual, sturdy leaves.
Layer two leaves together to form a firm base for each cup, and fill them by spooning approximately 2. 0 tbsp. (1. 0 oz / 28 g) of the mackerel and egg salad mixture into each lettuce cup before serving.
Check for Pin Bones: Even if you buy pre-cut mackerel fillets, always run your fingers down the centerline of the fish before grilling.
Remove any remaining pin bones with tweezers for a much more pleasant eating experience.
Add a Citrus Kick: Mackerel is a beautifully rich and oily fish that benefits greatly from a touch of acidity. A quick squeeze of fresh lemon juice over the fish right after grilling will brighten up the entire flavor profile of your dish.