Jalapeño Vinegar
¼ cup [60 ml] rice wine vinegar
2 Tbsp fresh lemon juice
2 Tbsp fresh orange juice
1 Tbsp sugar
½ tsp kosher salt
1 jalapeño chile, seeded and roughly chopped
¼ cup [60 ml] water

Roasted Garlic Purée
1 garlic head

1 tsp extra-virgin olive oil
½ tsp fresh lemon juice
Kosher salt

2 skin-on Spanish mackerel fillets (about 1 Ib [455 g) total weight)
2 tsp sesame seeds
Best-quality olive oil for drizzling
Flaky sea salt
3 spring onions, cut into thin strips

To make the jalapeño vinegar:
In a small saucepan over medium heat, combine the vinegar, lemon juice, orange juice, sugar, and kosher salt and stir until the sugar and salt dissolve.
Remove from the heat and add the jalapeño and water.
Transfer to a blender, and process on high speed until the jalapeño is liquefied.
Strain through a fine-mesh sieve into a small bowl and reserve.
(The vinegar can be made up to 1 day in advance. Store in an airtight glass jar in the refrigerator until ready to use.)

To make the garlic purée:
Preheat the oven to 375°F [190°C].
Wrap the garlic tightly in aluminum foil and roast until soft to the touch, about 1½ hours.
Allow to cool.
Unwrap the garlic and remove the papery skin.
In a small bowl, use a fork to mash the peeled garlic with the extra-virgin olive oil and lemon juice to make a chunky paste.
Season with kosher salt.
(The roasted garlic purée can be made up to 1 day in advance. Store in an airtight container in the refrigerator until ready to use.)

Turn the mackerel fillet skin-side up on your work surface and, working from the corner of the fillet, use your fingers to peel the topmost layer of skin back, leaving some of the brilliantly colored underlayer of skin behind.
Holding the fillet down with one hand, gently pull off the outer layer of skin completely.
You can also do this with a knife, but be careful not to break or damage the fish while doing so.
This is a bit tricky but very possible once you get the hang of it.
You can also just ask your fish guy to skin the fillets for you.
Slice the mackerel on a steep bias to get thin, even slices and arrange on a serving plate.
(You can prepare the fish 1 hour or so in advance. Cover with plastic wrap until ready to serve.)

In a small, dry frying pan over medium heat, toast the sesame seeds just until fragrant and beginning to brown, about 3 minutes.
Set aside and let cool.

Top each fish piece with a dab of the garlic purée and plenty of the jalapeño vinegar.
Drizzle with best-quality olive oil, sprinkle with sea salt, garnish with the green onions, and finish with a light sprinkle of sesame seeds.
Serve immediately.