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.