Preheat the oven to 400℉. Place potatoes on a baking sheet and place in the oven for 1 hour or until fork tender; place in the fridge overnight.

Cut your potatoes in half, then into thirds or quarters, depending on the size. Fill a large-heavy-bottomed pot halfway with vegetable oil. Heat to 360℉.

Fry your potatoes in 2 or 3 batches until crispy and golden brown. Remove from the oil using a spider, place on a baking sheet with a wire rack, and season with salt while hot.

Season fish with salt on all sides, and let it sit at room temperature for 8 minutes.

In a medium-size bowl, add flour, baking powder, salt, and whisk until combined; slowly whisk in beer until homogenous.

Pat dry your fish with a paper towel to remove the excess water. In a bowl, place flour for dredging; then toss a couple of fish pieces to coat, shake off the excess, put them in the batter, and then fry your fish for 3 - 4 minutes or until golden brown and fully cooked.

Drain on a wire rack, season with salt while hot, and repeat with the rest of your fish. Serve with your serrano tartar sauce, ketchup, and your potatoes.

In a blender, place serrano peppers, lemon juice, garlic, and water; blend until smooth. Add a few drops of oil and blend once again.

To a medium-sized bowl, add mayo, shallots, pickles, mustard, and your serrano puree; stir together and season with salt.