1. Blend hazelnuts and garlic until coarsely chopped. Add roasted peppers, parsley, vinegar, tomato paste, red pepper flakes, 1/2 cup olive oil, and 1 1/2 teaspoons paprika, until sauce is finely chopped. Season with salt, black pepper, and more red pepper flakes if needed.
2. Combine remaining 1 tablespoon olive oil and 1/2 teaspoon paprika in a small bowl. Brush paprika oil over cod and season with salt and black pepper.
3. Prepare a grill for medium-high heat and preheat for 5 minutes. Grill cod, covered, until charred and cooked through, about 6 minutes per side.
4. Spoon romesco sauce over cod, sprinkle with parsley, and serve with remaining sauce.