Grilled Chicken with Red Pepper-Pecan Romesco Sauce

2 medium red bell peppers
1 medium tomato
1 pound chicken cutlets
¾ teaspoon salt, divided
½ teaspoon ground pepper, divided
½ cup chopped pecans, toasted
1 clove garlic
2 tablespoons extra-virgin olive oil
1 tablespoon red-wine vinegar
¼ teaspoon crushed red pepper
Chopped scallions for garnish

Preheat grill to medium-high. Grill bell peppers and tomato, turning occasionally, until blistered all over and charred in spots, 12 to 15 minutes. Transfer to a plate and let rest until cool enough to handle, about 5 minutes.
Meanwhile, sprinkle chicken with 1/4 teaspoon each salt and pepper. Grill the chicken, flipping occasionally, until an instant-read thermometer inserted in the thickest part registers 165 degrees F, 6 to 8 minutes total. Transfer to a plate, tent with foil and let rest for 10 minutes.
Remove and discard skin and seeds from the peppers and tomato. Place the peppers and tomato in a blender and add pecans, garlic, oil, vinegar, crushed red pepper and the remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Puree on high until well combined, about 1 minute. Serve the chicken with the sauce and topped with scallions, if desired.