In a large container, whisk together 1/2 cup salt and 1 gallon (16 cups) water. Put the pork loin in the container and seal tightly. Brine the pork, refrigerated, for at least 24 hours before using.
To make the marinade, remove the stem and seeds from the Nora and ancho chiles. Toast the chiles in a dry pan until just fragrant but no color forms, about 2 minutes. Place the chiles and hot water in a blender and blend on high until smooth. Add the garlic, honey, thyme, rosemary, pepper, olive oil and 1 teaspoon salt and blend again until smooth. Let cool.
In a small bowl, blend together one-quarter of the marinade with the butter. Set aside at room temperature.
Remove the pork loin from the brine and pat dry. Using a knife-honing steel, hollow out the very center of the pork loin. Place the Bilbao sausage in the hole you have created. Spread the remaining marinade evenly on the stuffed pork loin and allow it to marinate at least 20 minutes and up to 2 hours.
Preheat the oven to 375 degrees F. Place the pork loin on a roasting rack and roast until the internal temperature reaches 140 degrees F, 45 minutes to 1 hour. Remove from the oven, brush the butter-marinade mixture all over the pork and allow to rest for at least 10 minutes before carving.
Plate on a large platter and garnish with toasted pine nuts, arugula leaves and Chimichurri if using. Serve with Tamal en Cazuela if desired.
In a blender, combine the white wine vinegar, chile flakes, bay leaves and garlic. Blend until smooth. Combine the mixture with the parsley, oregano and olive oil. Season to taste.
Tamal en Cazuela:
Melt 2 tablespoons of the butter in a saucepan over medium heat and cook the corn kernels until they are translucent, about 10 minutes. Add 5 cups of the chicken stock and bring to a boil over high heat. Slowly whisk in the cornmeal and continue stirring until the mixture begins to thicken, about a minute or so.
Decrease the heat to low and cook, stirring frequently, until the tamal is very thick and creamy, 35 to 40 minutes. Add the remaining 1/2 cup chicken stock and stir to incorporate. Add the cream and remaining 3 tablespoons butter and stir to combine. Season to taste with salt and pepper. Keep warm until serving time, or cover and refrigerate for up to 2 days and rewarm over medium-low heat, adding more stock as needed to restore the creamy texture.