The flavor of romesco improves with time; you can store it in the refrigerator for up to three days before serving, at room temperature.
Char peppers over the flame of a gas stove, turning with tongs, until blackened and blistered. (Or char under broiler on a rimmed baking sheet, turning as needed.) Transfer to a bowl, cover with a plate, and let steam 15 minutes. Remove skins, using paper towels (and a paring knife for any stubborn spots); discard. Remove and discard stems, ribs, and seeds.
Transfer peppers to a food processor. Add garlic, almonds, vinegar, tomato paste, and pimenton. Pulse until combined. With machine running, slowly add oil, processing until combined. Season with salt.