Preheat the oven to 400°F (200°C).
Peel and core the apples.
Remove the rosemary leaves from the sprigs and finely chop them.
Mix the chopped fresh rosemary with the apples.
Use half of the apples to stuff the duck cavity.
Tie the duck legs together with kitchen twine to keep the apples inside.
Using a very sharp knife, make small incisions on the duck breasts — this helps render the fat and create crispy skin.
Season the duck all over with salt and dry rosemary.
Place the duck breast side down in an oval Dutch oven (or any deep oven-safe pan that fits the whole duck and can be covered).
Roast uncovered for 15 minutes.
Reduce the oven temperature to 350°F (175°C), cover the pot, and roast for 1 hour.
Carefully remove the duck from the oven using oven mitts.
Flip the duck breast side up (two large forks work well — just be careful not to splash hot fat).
Cover and roast for another 45 minutes.
Transfer the duck to a baking sheet fitted with a rack and brush with some duck fat.
Increase the oven temperature back to 400°F (200°C) and roast the duck for 15–20 minutes, until the skin is crispy.