Step 1 Preheat a gas grill to high (450°F to 550°F), or push hot coals to one side of a charcoal grill. Pat fish dry and season with 3/4 teaspoon salt and black pepper to taste. Let seasoned fish rest while grill preheats. Place onion, garlic, cilantro, parsley, jalapeño, and remaining 1/4 teaspoon salt in a food processor; process until smooth. Add lemon juice and 1/4 cup olive oil; pulse until combined, about 2 times.
Step 2 When grill is hot, brush fillets with remaining 1 tablespoon olive oil. Place fillets, skin side down, parallel with the grill grates over one side of grill (or over the hot coals on a charcoal grill). Grill, uncovered, about 1 minute. Turn off burner under fillets, leaving the opposite burner on high. (Or rotate charcoal grill grate 180 degrees using tongs and an oven mitt so fillets sit opposite the coals.) Do not flip fish. Grill, covered, until cooked through, 3 to 5 minutes per inch of thickness. Using a fish spatula, transfer fillets to a platter. Serve with chimichurri.