Bring a large pot of salted water to aboil. Add the chard and cook until tender, 8 to 10 minutes; remove with tongs and set aside in a colander to cool. Return the water to a boil. Add the rice and cook until al dente, 15 to 20 minutes; drain and set aside. Squeeze out the excess moisture from the chard and roughly chop.
Preheat the oven to 375 degrees F and butter a 3-quart baking dish. Pulse the pancetta, garlic, rosemary, red pepper flakes and 1 teaspoon salt in a food processor to make a coarse paste. Transfer to a bowl and add the rice, chard, gruyere, raisins, scallions, basil, parsley, lemon zest and nutmeg. Add the beaten eggs and toss.
Transfer the rice mixture to the prepared baking dish. Cover with aluminum foil and bake until heated through, 50 minutes to 1 hour.