Special equipment: a deep-fryer or a Dutch oven with a deep-fry thermometer
For the saffron tartar sauce: Bring the vinegar to a simmer in a small saucepan, add the saffron, remove from the heat and let steep for 5 minutes. Transfer to a small bowl and refrigerate until cool, about 10 minutes.
Put the mayonnaise in a medium bowl and whisk in the onion, mustard, capers, cornichons, lemon zest, lemon juice and the cooled saffron vinegar mixture. Stir until combined and season with salt and pepper. Cover and refrigerate for at least 30 minutes and up to 8 hours to allow the flavors to meld. Stir in the parsley just before serving.
For the sherry vinegar tartar sauce and chips: Whisk together the vinegar and mustard in a small bowl and season with salt and pepper. Slowly whisk in the olive oil until emulsified and stir in the tarragon. Let sit at room temperature for at least 15 minutes and up to 60 minutes before serving to allow the flavors to meld.
Set a baking rack over a baking sheet and set aside. Heat the vegetable oil in a deep fryer or medium Dutch oven over medium heat until it registers 325 degrees F.
Fry the potatoes, in batches, until pale golden blonde and slightly soft, about 5 minutes. Remove with a slotted spoon to the prepared baking sheet. Repeat with the remaining potatoes. Increase the heat of the oil to 365 degrees F.
Fry the potatoes again, in batches, until golden brown and crispy, about 2 minutes. Drain on a plate lined with paper towels, and season immediately with more salt and the smoked paprika. Reserve the oil for frying the fish.
For the fried parsley leaves: If desired for serving, fry the parsley leaves in the oil until crisp, about 30 seconds. Drain on a plate lined with a paper towel, season with salt and set aside for serving.
For the fried fish: Add more oil to the the deep fryer or Dutch oven if necessary and heat to 365 degrees F. Set a baking rack over a baking sheet and set aside.
Whisk together the beer, rice flour, breadcrumbs and salt and pepper until smooth, adding more beer if needed. The batter should be just thick enough to coat the back of a spoon.
Whisk together the flour, cayenne and some salt and pepper on a plate. Season the halibut on both sides with salt and pepper, dredge in the seasoned flour and tap off the excess.
Dip the fish in the batter and let the excess run off. Fry the fish in batches, until golden brown on both sides and cooked through, about 5 minutes. Transfer to the prepared baking sheet and season with salt. Serve with the saffron tartar sauce, sherry vinegar and tartar sauce and chips. Garnish with the fried parsley, if desired.