Gratineed Gnocchi with Spinach and Ricotta

1 (1-lb) package potato gnocchi
2/3 cup heavy cream
1/2 teaspoon all-purpose flour
1/2 teaspoon salt
1/2 teaspoon black pepper
1/8 teaspoon ground nutmeg
3 (5-oz) packages baby spinach
1/2 cup whole-milk or part-skim ricotta
2/3 cup shredded mozzarella

Preparation
Preheat broiler.
Cook gnocchi in a 5-quart pot of boiling salted water according to package instructions (gnocchi will float to surface when done). Drain in a colander.
Whisk together cream, flour, salt, pepper, and nutmeg in a 12-inch ovenproof skillet, then bring to a boil over moderate heat, whisking. Continue to boil, whisking frequently, until reduced by half, about 2 minutes. Add spinach in handfuls, tossing with tongs, and cook until wilted, 2 to 4 minutes. Remove from heat, then stir in gnocchi. Spoon ricotta over gnocchi in 5 large dollops and sprinkle with mozzarella. Broil 4 to 6 inches from heat until cheese is browned and bubbling in spots, about 2 minutes. Season with pepper.