After your fish has thawed, salt the fillet liberally and let rest 6-8 minutes, then rinse in ice water and immediately pat dry. This vastly improves the texture of previously frozen fish by pulling excess moisture out.
Remove skin and pin bones from your mackerel fillet(s) and cut into small pieces. If you have any aversion to “fishiness” go for a rough mince, otherwise a dice or chop to your desired consistency.
On a separate cutting board, remove the skin from your beets and break them down into a medium dice.
In a bowl, whisk the remaining ingredients, reserving some lemon zest for a garnish.
Separately, dress the chopped mackerel and the diced beets with your dressing.
Using a round mold, a cookie cutter or an oiled ramekin, layer your dressed ingredients and plate. Serve with rye toast, crostini or crackers.