For the filling: Bring a large pot of water to a boil. Meanwhile, fill a large bowl one-third full with ice. Add cold water to cover the ice. Set a colander into the bowl.
In batches, cook the spinach in the boiling water just until bright green, about 1 minute. With tongs, immediately plunge the spinach into the colander to stop cooking and set the color. When all of the spinach is cooked and cooled, lift the colander from the water and squeeze out the excess water from the spinach. Chop the spinach and set aside.
Melt the butter in a deep skillet or broad saucepan over medium heat. Add the onions and cook until soft, 3 to 5 minutes. Add the garlic; cook an additional minute, until toasted and fragrant. Add the cream, the blanched spinach and 1/2 teaspoon salt. Bring to a boil and cook until the cream has cooked down and thickened, about 15 minutes. Stir in the Parmesan, feta, white pepper, nutmeg and more salt to taste. Stir to blend, then remove from the heat.
For the crust: Preheat the oven to 350 degrees F.
Unroll the phyllo and cover it with a damp kitchen towel; keep the pastry covered while working to keep in the moisture. Cut the phyllo in half crosswise.
Using a pastry brush, coat six 4 1/2-inch tart pans with melted butter. Gently lay a "square" of phyllo in one of the pans, letting it fall loosely into place. Brush with butter, covering the pastry sheet completely. Repeat with five additional sheets. Using kitchen shears, trim the excess dough, leaving a good 1 inch of overhang. Shape the phyllo around the perimeter of the tart pan to create an attractive, rustic edge. Repeat for the remaining five pastry tarts. Fill each tart with the creamed spinach mixture.
Place the filled tart pans on a baking sheet. Bake on the center oven rack until the crust is golden brown, 30 minutes.