To a small saute pan, add 1/2 cup of the canola oil and the annatto seeds. Turn the heat to medium and allow the annatto seeds to color the oil, stirring often, 2 to 3 minutes. Strain and reserve the oil.
To a food processor, add the green onions, garlic, lime juice and zest, jalapeno, cilantro, cumin, chile flakes and some salt and pepper. Turn the food processor on and stream in the annatto-colored oil plus the remaining 1/2 cup canola oil. Process until thoroughly combined.
Put the halibut fillets into a resealable bag and pour in the marinade. Place in the refrigerator and marinate for 20 minutes before grilling.
Preheat the grill to medium-high heat. Remove the halibut from the marinade and place on the hot grill, 5 to 8 minutes per side.
Butter the insides of the ciabatta rolls and place them buttered-side down on the grill, then flip. Grill the rolls until both sides are lightly toasted and crispy around the edges. Smear the inside bottoms and tops of bread with the Chorizo Mayo. On the bottom slice of bread, put a layer of pickles and red onions. Place the halibut on the onions, then top with some tomato, cabbage and the top bun. Repeat for the remaining sandwiches.
Kosher salt and freshly ground black pepper
Remove the chorizo from the casing. Heat the canola oil in a nonstick pan over medium heat and add the chorizo, breaking it up into small pieces with a wooden spoon as it cooks. When the sausage has browned, and the fat had rendered and the oil in pan is red, about 7 minutes, then remove from the heat and cool.
In a mixing bowl, combine the chorizo and oil from the pan with the mayo, garlic and scallions. Season with the cilantro, lemon zest and juice and some salt and pepper. Place in the fridge until ready to use.
Annatto adds a great orange hue to the halibut filets. If you don't have any, don't let that stop you from making this sandwich! Just skip the step of heating the oil, and add a pinch of paprika to the food processor.