Special equipment: kitchen twine
For the lamb: Preheat the oven to 400 degrees F.
Soak the guajillo chiles in warm water to soften. Puree with 1 to 2 tablespoons of the soaking liquid in a small food processor or blender to make a paste.
In a small bowl, combine the guajillo paste, fennel, parsley, rosemary, thyme, olive oil, garlic and 1 teaspoon each kosher salt and pepper.
Cut the lamb lengthwise, stopping 1/2 inch before going all the way through. Open the lamb like a book and spread with the fresh herb and chile mixture. Close the lamb and tie with kitchen twine at 1-inch intervals. Season the outside heavily with salt and pepper.
Put the lamb on a baking sheet and roast until golden brown, 30 to 40 minutes. Remove from the oven and lower the temperature to 300 degrees F. Transfer the lamb to a Dutch oven or ovenproof pot, add the chicken broth and wine and cover. Return the lamb to the oven and braise until the internal temperature reaches 135 degrees F, about 1 hour. Transfer the lamb to a cutting board (reserve the liquid in the pot) and let rest for 20 minutes before slicing.
For the roasted potatoes: Meanwhile, toss the potatoes with the olive oil and sea salt in a roasting pan. Roast until tender, about 40 minutes. Remove from the oven, smash each potato to flatten slightly and then cook 10 more minutes.
Pour the reserved lamb-braising liquid in a saucepan, set over medium-high heat and reduce by half. Whisk in the flour while boiling to thicken. Season with salt and pepper.
Serve the lamb and roasted potatoes with the sauce.
Recipe courtesy of Marcela Valladolid