Fill a very large pot three-quarters full with water and bring to a boil. Add the collard greens to the water and make sure they are all well submerged. Return the water to a boil and simmer until the greens are tender and bright green, 10 to 15 minutes. Drain the greens and run under cold water to stop the cooking. Drain the greens well, squeezing out excess water with your hands or by placing in a kitchen towel and squeezing. Chop the greens finely and set aside.
Melt the butter in a skillet or Dutch oven large enough to hold all of the greens. Add the garlic and shallots and cook over medium heat until softened but not browned, 4 to 6 minutes. Add the cream and bring to a simmer, stirring occasionally. Cook until reduced by half, about 25 minutes - watch carefully as the cream can quickly boil over! Add the greens and toss until warmed through. Season generously with nutmeg, salt and pepper. Add to a serving dish and sprinkle with the Parmesan.