For the salsa verde, finely chop the garlic, herbs, capers, gherkins and anchovies on a large board. Place in a bowl, add the mustard and vinegar, then slowly stir in the oil until you achieve the right consistency
Heat a griddle pan until hot, season the mackerel with salt and drizzle with a little olive oil until coated. Place the mackerel skin, side down, in the pan. Cook for 2-3 minutes then turn and cook for another few minutes until cooked through. Remove from the pan, place onto 4 warmed plates and squeeze over the lemon juice.
Wipe the pan quickly, add the olive oil and place the baby gem in the pan, cut side down. Turn after 30 seconds or until charred.
Serve the mackerel with the charred baby gem and the salsa verde.