Chop enough cilantro stems to measure 1/2 cup, then transfer to a food processor along with peanuts, ginger, garlic, pepper, 2 tablespoons oil, 1 tablespoon lime juice, 1 teaspoon salt, and 1/2 teaspoon chile. Process until finely chopped, about 30 seconds, then transfer to a bowl. Do not clean processor.
Pat fillets dry and arrange, flesh sides up, tightly on rack of broiler pan (fillets should just touch). Spread peanut mixture evenly over tops of fillets.
Broil fish 3 to 4 inches from heat until lightly browned and just cooked through, about 4 minutes.
While fish broils, chop enough cilantro leaves to measure 1 cup, then pulse in processor along with pineapple, fish sauce, remaining tablespoon each of oil and lime juice, remaining 1/4 teaspoon salt, and remaining 1/2 teaspoon chile until combined.
