Bring a very large stockpot of water to a boil. Prepare a large bowl of ice water. Add half of the mustard greens to the boiling water, a little at a time, and push them down into the water. Let the water return to a boil, and then cook until tender, about 10 minutes. Using tongs, transfer the greens to the bowl with the ice water to reserve. Repeat with the remaining greens. Drain the greens very well and squeeze them dry in a kitchen towel. Chop finely and set aside.

In a large high-sided skillet, heat the grapeseed oil.  Add the onions and garlic and cook over moderate heat until softened, about 6 minutes. Stir in the half and half and the Neufchatel and simmer; stir occasionally, until reduced by half, about 20 minutes. Add the greens to the cream mixture and toss until warmed through, about 3 minutes. Season generously with nutmeg, salt and pepper.