Place the dried ñora or chilli in a small heatproof bowl, cover it with just-boiled water and leave it to soften for 20 minutes. Remove the stem and seeds once softened.
Make the mojo verde. Place the coriander, green pepper, garlic, cumin, salt and vinegar in a small food processor. Blend to a coarse paste. With the motor running, gradually add the olive oil. Add enough cold water to make a loose, spoonable sauce. Taste and adjust the salt or vinegar, then transfer it to a small bowl.
Make the mojo rojo. Place the softened pepper, garlic, cumin, paprika, optional cayenne, salt and vinegar in the cleaned processor. Blend to a paste, then gradually add the olive oil. Thin it with a little cold water until spoonable. Transfer it to a second bowl. Leave both sauces at room temperature while preparing the fish and potatoes.
Preheat the oven to 220°C/425°F, or 200°C fan. Line a large roasting tray with baking parchment.
Dry the fish thoroughly inside and out. Make three shallow diagonal cuts on each side, cutting through the skin but not as far as the backbone. Rub the fish with the olive oil and sea salt, including inside the cavity.
Cut half the lemon into thin slices and place them inside the fish with the coriander stems. Reserve the remaining lemon for serving. Place the fish on the prepared tray and roast it for 28–35 minutes.
As soon as the fish goes into the oven, scrub the potatoes without peeling them. Place them in a saucepan in a snug single or double layer. Add the coarse salt and lemon slice, then pour in only enough cold water to almost cover the potatoes.
Bring the potatoes to a boil, reduce the heat and simmer for 20–25 minutes, or until a small knife enters the centre easily. The cooking time will depend on their size.
Drain away all the potato water. Return the uncovered pan to low heat and shake it frequently for 1–2 minutes, until the skins look dry, lightly wrinkled and dusted with salt. Remove the pan from the heat, cover it with a clean kitchen towel and leave the potatoes for 5 minutes.
Begin checking the fish after 28 minutes. It is ready when the flesh at the thickest part is opaque, separates easily from the backbone and reaches 63°C/145°F. Remove it from the oven and let it rest for 5 minutes.
Transfer the fish to a large serving platter and arrange the papas arrugadas around it. Spoon a little mojo verde over the fish and serve the remaining green and red mojo in separate bowls. Add the reserved lemon wedges and serve immediately.