Special equipment: A 9 1/2-inch deep-dish pie plate
Preheat the oven to 425 degrees F. Butter a 9 1/2-inch deep-dish pie plate.
Cook 1/3 of the spinach, 1 cup water, the oil and 1/2 teaspoon salt in a large pot over medium heat, stirring with tongs, until the spinach is wilted and tender; add the remaining spinach in 2 batches, stirring until wilted and tender, about 15 minutes total. Transfer the spinach to a colander. When it is cool enough to handle, squeeze out as much liquid as possible with your hands or a wooden spoon; coarsely chop, and set aside. (The spinach can be prepared up to this point up to 1 day in advance.)
Wipe out the pot. Add 4 tablespoons of the butter, and melt over medium heat. Add the onions, garlic, 1 teaspoon salt and a few grinds of pepper, and cook, stirring occasionally, until the onions are tender, about 10 minutes. Add the cream, Worcestershire and nutmeg, bring to a simmer and reduce until very thick, 8 to 10 minutes. Remove the pot from the heat, and add the spinach. Transfer the mixture to the prepared pie plate.
Melt the remaining 4 tablespoons of butter. Place 1 sheet of phyllo on a cutting board; keep the rest covered with a damp towel. Cut the sheet in half crosswise. Use a pastry brush to coat each half with melted butter; drape 1 half on top of the creamed spinach, then the other on top of the first, scrunching the edges to fit. Repeat with the remaining sheets of phyllo. Brush any remaining butter over the edges of the phyllo. Cover the pie plate loosely with foil.
Bake, covered, for 10 minutes. Remove the foil, and continue baking until the phyllo is golden brown and the filling is bubbling, 15 to 18 minutes more.
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