Heat olive oil in a large skillet and sauté the garlic and onion briefly, add the greens, cover and cook over medium heat until soft, adding 1/2 cup water to help steam them until done.
Beat the eggs in a large bowl, combine with milk, Besciamella Sauce, Parmigiano-Reggiano and salt, pepper and nutmeg, to taste. Stir in the cooked greens and pour into a baking dish that has been buttered and dusted with breadcrumbs. Top with a light coating of bread crumbs.
Bake at 375° until firm and brown on the edges. Let the Sformato sit for 10 minutes before slicing and serving.
Melt the butter in a small sauce pan and whisk in the flour; allow the flour to gently sauté until it is covered all over with bubbles, then vigorously whisk in the hot milk. Add 1 teaspoon salt, a grinding of pepper and nutmeg. Allow to simmer 5 minutes, cool and use.