Heat oven to 450° F. In a small bowl, mix 3 tablespoons of the oil with the garlic, sage, and 1 teaspoon each salt and pepper.
Finely chop 1 apple. In a 9-by-13-inch baking dish, toss the fennel and chopped apple with the remaining 1 tablespoon of oil and 1 tablespoon of the lemon juice. Rub the pork all over with the sage oil. Place the pork on top of the fennel mixture. Roast for 30 minutes, until the pork is golden brown. Cover, reduce oven temperature to 350° F, and roast for about 3 hours more, until very tender.
Transfer the roast to a cutting board and let rest for 15 minutes. Thinly slice crosswise.
Skim the fat off the top of the dish (about 2 tablespoons). Thinly slice the remaining apple. Add the watercress, sliced apple, ¼ teaspoon each salt and pepper, and the remaining 1 tablespoon of lemon juice to the casserole and gently toss. Return the pork to the dish and serve warm with lemon wedges.