Prepare Romesco sauce: In a food processor, grind the almonds. Add roasted peppers, garlic, bread, parsley, and hot pepper flakes. Blend until it becomes a paste. Add the vinegar and pulse to blend. With the motor running, gradually pour the olive oil through the feed tube in a steady stream until the mixture thickens like mayonnaise. Season to taste with salt and black pepper. Store in a covered container in the refrigerator for up to 3 days.

Preparing the sandwich:

Prepare a hot fire in your grill.

Brush the cut sides of the ciabatta with olive oil. Brush the chicken breasts with olive oil on both sides and season with salt and pepper.

Grill the chicken, turning once, for 5 minutes total. Grill the ciabatta, cut sides down, until it has good grill marks, 1 to 2 minutes.

Slather all the grilled sides of the ciabatta with the Romesco. Place the chicken breasts on the bottom half of the bread, shave some Manchego cheese over the chicken, then top with baby greens and the top half of the bread. If you’re using a loaf of ciabatta, you’ll have 2 chicken breasts on each loaf, so cut each loaf in half horizontally. If you’re using ciabattini, you’ll use 1 breast for each roll. Serve immediately.