¼ teaspoon freshly ground black pepper, plus more for seasoning
2 large jarred fire-roasted peppers, rinsed and patted dry
⅔ cup roasted salted almonds
1 tablespoon sherry or red wine vinegar, or more to taste
1 ½ teaspoons smoked paprika
4 whole scallions (greens and whites), halved lengthwise
4 thin-cut chicken breasts (about 1 ½ pounds)
Preheat the oven to 375°F. Arrange the tomatoes and onions, cut sides up, aloing with the garlic cloves on a roasting pan. Drizzle with 3 tablespoons olive oil and season generously with salt and pepper. Roast until the tomatoes and onions are wilted and the garlic is soft, 30 to 35 minutes.
Remove from the oven, cool, then squeeze the garlic directly into the bowl of a food processor along with the roasted tomato and onion. Add the peppers, almonds, vinegar, smoked paprika, and the 1 teaspoon salt and ¼ teaspoon pepper, turn on the processor and, with the motor running drizzle in the ½ cup olive oil until a chunky-smooth sauce forms, 15 to 20 seconds. Season with more salt and vinegar to taste.
Preheat a grill or grill pan over medium-high heat. Brush the chicken and scallions with the remaining 2 tablespoons of the oil and season generously with salt and pepper. Working in batches, grill until the chicken forms grill marks and the scallions are lightly charred, 2 to 3 minutes per side. Spread 1 cup of the romesco on a serving platter and top with the chicken breasts. Season with salt and pepper Chop the charred scallions and scatter them on top of the chicken.Leftover romesco can be refrigerated for 4 to 5 days. Serve as a dip, or with grilled meat or fish.