Step 1

Preheat oven to 300°. Coarsely grind fennel seeds and red pepper flakes in a spice mill or with a mortar and pestle. Combine spice mixture, chopped marjoram, chopped thyme, 1 Tbsp. salt, ½ tsp. pepper, and 3 Tbsp. oil in a small bowl. Rub chicken inside and out with spice mixture. Stuff chicken with lemon, garlic, 2 marjoram sprigs, and 2 thyme sprigs. Tie legs together with kitchen twine.

Step 2

Toss potatoes with remaining 3 Tbsp. oil on a rimmed baking sheet; season with salt and pepper. Push potatoes to edges of baking sheet and scatter remaining 2 marjoram and 2 thyme sprigs in center; place chicken on herbs. Roast, turning potatoes and basting chicken every hour, until skin is browned, meat is extremely tender, and potatoes are golden brown and very soft, about 3 hours. Let chicken rest at least 10 minutes before carving.