How to Make It
Stir together 1 quart water and 1/3 cup salt in a medium bowl until salt is dissolved. Add fish; cover and chill 30 minutes. Remove fish and pat dry; discard salt water. Remove bloodline from fish; cut fish into 1/2-inch pieces. Toss with 2 tablespoons oil and garlic. Cover and chill 1 to 2 hours. (Let fish stand at room temperature 20 minutes before cooking pasta.)
Heat a large cast-iron skillet over high until smoking. Toss peppers with 2 tablespoons oil; working in 3 batches, cook until blistered on all sides, about 3 minutes. Transfer to a paper towel–lined baking sheet. Let cool 10 minutes. Cut peppers into 1/4-inch rings; discard stems. Using your hands, toss peppers to remove as many seeds as possible. Transfer peppers to a medium bowl, and toss with 2 tablespoons oil and 1 teaspoon salt.
Bring remaining 4 quarts water and 1/3 cup salt to a boil in a large pot over high. Add pasta, and cook, stirring occasionally, until al dente, 11 to 12 minutes. Drain, reserving 1 cup cooking liquid.
Return large pot to medium heat; add 2 tablespoons olive oil. Add half of fish pieces in a single layer; cook, stirring occasionally, until cooked through, 3 to 4 minutes. Set aside. Repeat with 2 tablespoons oil and remaining fish. Return all fish to pot; add peppers, pasta, and 2/3 cup reserved cooking liquid. Using tongs, lift and stir pasta gently until liquid thickens, about 2 minutes, adding remaining 1/3 cup reserved cooking liquid if needed. Remove from heat; gently stir in colatura, parsley, lemon juice, remaining 1/4 cup oil, and remaining 1 teaspoon salt; toss gently to coat. Transfer pasta to a serving dish; garnish with black pepper.