Place spinach in a large pot over high heat. Cook, covered, with just the water clinging to leaves, stirring occasionally, until wilted, 8 minutes. Transfer to a colander to drain, squeezing out and reserving excess liquid. Roughly chop spinach; set aside.

In a medium skillet, melt butter over medium heat. Add flour, and cook, stirring, 1 to 2 minutes. Slowly whisk in milk; season with salt, pepper, and sugar. Thin with reserved spinach liquid. Stir in spinach. Sprinkle with nutmeg to taste and, if desired, top with a dollop of sour cream. Serve immediately.