1) Whisk together the salt, ancho and chili de arbol powders in a bowl. Set aside.
2) Cut potatoes into 1-cm thick slices then cut each slice into 1-cm thick fries. Place chips in a large bowl of cold water.
3) Heat oil in a large, straight sided skillet to 160C. Drain chips in batches on paper towels. Fry each batch for 3 to 4 minutes until a pale blonde colour forms and remove to a sheet pan lined with paper towels.
4) Just before serving, increase the heat of the oil to 190C and fry the potatoes again, in batches, until golden brown. Remove to a sheet pan lined with paper towels and season with the spices and chopped coriander.
For the fish:
5) Preheat a deep-fryer or a deep pot filled halfway with oil to 185C.
6) Whisk together the flour, 1 teaspoon salt, 1/2 teaspoon pepper and beer in a large bowl. Set aside for 10 minutes. Fold in the egg whites.
7) Preheat oven to 150C/Gas Mark 2. Season the fish with salt and pepper, dredge in flour, tap off excess then dip the fish in the batter and fry in batches until golden brown on both sides. Remove with a slotted spoon to a plate lined with paper towels. Transfer to a baking sheet and keep warm in the oven. Serve with, Chips, Lemon-Habanero Tartar Sauce and Serrano Vinegar.
Lemon-Habanero Tartar Sauce:
8) Bring the lemon juice to a boil over high heat in a small saucepan and cook until reduced to 125ml. Let cool.
9) Combine cooled lemon juice, mayonnaise, anchovy and habanero in a food processor and process until combined. Scrape the mixture into a medium bowl and fold in the cornichon and capers and season with salt and pepper. Cover and refrigerate for at least 1 hour before serving.
10) Bring vinegar to a simmer in small saucepan. Add the chillies and salt, remove from the heat and transfer to a glass bottle or jar. Let sit at room temperature for at least 8 hours.