1. The salt cod. The day before serving, placed the cod in a large bowl, cover with cold water and refrigerate. Every few hours, change the water, to leach out as much salt as possible.
2. The tomato mixture. Roast the tomatoes on a baking sheet 4 inches below a very hot broiler until blackened on one side, about 6 minutes, then flip them and roast the other side. Cool, then pull off the blackened skins. Puree tomatoes (and any accumulated juices) in a food processor or blender.
Heat the olive oil in a large (4-quart) saucepan over medium. Add the onions and cook, stirring frequently, until richly browned, about 10 minutes. Add the garlic and cook 2 minutes longer, then add the tomatoes. Cook, stirring occasionally, until the mixture is reduced and thick, about 15 minutes.
3. Simmering the bacalao. While the tomato mixture is reducing, drain the cod and place it in a medium-size (2- to 3-quart) saucepan, cover with fresh water and bring to a boil. Simmer 5 minutes over medium heat, then pour off the water.
Add the parsley, olives, jalapeños, marjoram, bay and thyme to the tomato mixture. Stir in enough water (usually about /12 cup) to give it a soupy consistency, then nestle in the cod and simmer, partially covered, over medium-low or about an hour, until the code has broken up into small chunks.
Break up any large pieces of cod in the simmering mixture, then stir in the diced potatoes and simmer until they're tender and the mixture is thick enough to hold its shape well with a spoon, about 15 minutes. Season with salt if necessary (the saltiness of the fish and the olives usually supplies ample seasoning), garnish with parsley and scoop into warm tortillas.