Pat the duck legs dry with a paper towel.
In a small bowl, combine salt, pepper and rosemary. Sprinkle duck generously with mixture. Place duck legs on a pan, cover tightly with plastic wrap and refrigerate overnight, for 24 hours.
The next day, pre-heat oven to 325 F degrees. Place duck legs, fat side down, in an ovenproof skillet, in a single layer (you may have to use 2 skillets or cook them in batches). Heat duck legs over medium-high heat until fat starts to render and skin turns a deep golden brown, about 20 minutes. Carefully flip duck legs over, skin side up, cover pan with aluminum foil and transfer skillet to the oven.
While the duck is in the oven, combine the cranberries, orange juice and zest, maple syrup, chili flakes salt and pepper in a bowl. Set aside.
Roast legs for 2 hours, covered, then remove skillet from oven and transfer duck legs to a cutting board or plate. Place skillet back on the stove over medium heat. Add garlic head(s), cut side down, to the pan, and cook undisturbed for 5 minutes, until beginning to brown and caramelize; take garlic head(s) out and set aside. Add the minced rosemary to the pan and cook, stirring until fragrant, about 1 minutes. Stir in the cranberry mixture and cook for 2-3 minutes. Turn the heat off and nestle the duck back in, add the garlic head(s), cut side up, the rosemary sprigs on top and slide the pan back into the oven, uncovered.
Continue roasting until duck is golden brown and at the fork tender and can be easily pulled from the bone, 1 hour more. Remove from the oven and enjoy.