To Make the Romesco Sauce:
- Heat oven to 400° F. Line a baking sheet with parchment or a silicone baking mat.
- Place peppers, tomatoes, garlic and almonds on the baking sheet and roast for 40 minutes, checking every 10-15 minutes to rotate peppers and tomatoes so that all sides get blistered, and to remove almonds and garlic once they are done (about 10-15 minutes in).
- Place peppers and tomatoes in a bowl and cover with foil or plastic wrap. Let sit for 10 minutes to steam (this makes them easier to peel).
- Gently peel skins away from peppers and tomatoes. Discard skins and pepper seeds and stem. Squeeze garlic from the cloves.
- Place all ingredients for the sauce in a high powered blender, such as a Vitamix, and blend until smooth.
For the Chicken:
- While vegetables are roasting, marinate chicken. Make sure chicken is an even thickness so that they cook more evenly. You may have to lightly pound them into even thickness. Sprinkle chicken breasts with salt, pepper and garlic powder on both sides. Add Worcestershire sauce, balsamic vinegar, and olive oil, and turn chicken to coat. Marinate in the refrigerator until you're ready to cook it.
- Heat a grill or grill pan over medium-high heat (about 400°F.) Cook chicken for about 5 minutes per side, then turn heat to low and/or move chicken to top rack of the grill and continue cooking until the internal temperature reaches 165° F and juices run clear, about 10-20 minutes, depending on the thickness of the chicken.
- Serve each chicken breast topped with Romesco sauce.