First, a word about timing: the brisket will cook for in the oven 4 hours, and the potatoes will bake with it for the last hour. When the 4 hours is done, the brisket gets removed from the oven, and the heat is increased to finish roasting the potatoes in a much hotter oven while the brisket rests before you carve it.
To cook the brisket in its adobo, I used a big, cast-iron Dutch oven with a good lid and a sheet of heavy silver foil to ensure a particularly good seal between the pot and its cover.
Start by setting your oven to 340F / 170C and positioning a shelf so that your big pot will sit in the middle of the oven.
Add the pasilla chilies to a heatproof bowl or jug and cover them with 1 ½ cups boiling water.
After the pasillas have soaked for 15 minutes, remove any stems and add them - seeds and all - to your food processor with the water.
Blitz the pasillas for a minute, then add the cayenne peppers, oregano, cumin, cinnamon, cloves, vinegar, salt, black pepper, and the juice, zest, and flesh of the orange. Now blitz the lot until you have a smooth, free-flowing paste.
Place the brisket in a large mixing bowl and pour the paste over it. Use your fingers to give the brisket a thorough coating of the paste.
Add the onions, garlic, and bay leaves to your Dutch oven and set the coated brisket - bone-side down - on top. Spoon any remaining paste over the top and sides of the brisket.
Cover your big pot with a layer of silver foil extending about 1 ½ inches beyond the pot’s rim. Sit the lid snugly on top and use your fingers to crimp the foil into place to create a tight seal all around the lid. That tight seal is important because it will ensure all the flavors of the brisket and adobo remain locked in the pot during its time in the oven.
Set your big pot in the oven and let it cook for 1 ½ hours at 340F / 170C. Then remove it from the oven so you can give the adobo a few stirs and baste the top and sides of the brisket with it. Once that’s done, carefully re-seal the pot with its foil and lid, and return it to your oven for another 2 ½ hours at the same 340F / 170C.
After 2 ½ hours, remove the pot from the oven and let it sit, without its covering of foil but with the lid ajar, for 15 minutes. This little bit of ‘resting’ matters because it allows the brisket to firm slightly so that it can be more easily carved into even, steak-like slices. Once that time’s up, carefully transfer the brisket to a board ready to be carved -and sauced.
To sauce your carved brisket, warm the adobo in your Dutch oven over medium-high heat. As soon as it starts bubbling, spoon a little of it over the cut brisket. I poured the remaining sauce into a warmed jug so that folks could help themselves more at the table.
Now, if you feel the adobo sauce is a little too thick, by all means, stir in some water as you heat it, ready for saucing the brisket and serving.