Halve the peppers lengthwise, remove the seeds and membranes, and cut each half lengthwise into several pieces, not too wide. Thinly slice the onion.
Put the onion in a pan with the lard and sauté over medium heat for 5 minutes. When it is almost translucent, add the peppers and season with salt and pepper. Cook over medium heat for 7 to 8 minutes. Meanwhile, cut the guanciale in