Prepare the Guisquil: Wash the guisquil thoroughly. You can peel them if you prefer a smoother texture, but it's not strictly necessary. Cut each guisquil in half lengthwise, then into quarters. Remove the seed from the center of each quarter. Slice the guisquil quarters into about 1/4-inch thick pieces.
Sauté Aromatics: Heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook until softened and translucent, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.
Cook the Guisquil: Add the sliced guisquil to the skillet. Season with salt, pepper, and optional red pepper flakes. Stir well to coat the guisquil with the oil and aromatics. Cover the skillet and cook for about 10-15 minutes, stirring occasionally, until the guisquil is tender but still holds its shape. You can test for doneness by piercing a piece with a fork.
Adjust Seasoning and Serve: Taste and adjust the salt and pepper as needed. If using, stir in fresh chopped cilantro or parsley just before serving. Serve hot.