Defrost the mackerel and pat dry with a kitchen towel.

Score the mackerel and season with pepper. Let it marinate for 10 minutes.

Lightly coat the mackerel with flour.

Heat oil in a pan and fry the mackerel on high heat, then reduce to medium heat.

Press the mackerel gently with a spatula to cook evenly.

Remove excess oil using a kitchen towel.

Slice the lemon and squeeze out the juice.

Clean the pan and melt butter with lemon juice. Add a pinch of sugar and simmer for 3–5 minutes.

Pour the butter lemon sauce over the mackerel.

Toast the baguette in the same pan used for the sauce.

Mash the avocado and spread it on the toasted baguette.