Ingredient

• 12 cups water
• 2 tablespoons plus 1 teaspoon. kosher salt, divided
• 3 (10-ounce) package fresh spinach (or 1 [2-pound] bunch spinach, trimmed)
• ¼ cup unsalted butter
• ½ cup finely chopped yellow onion (from 1 small [7 ounce] onion)
• 4 small garlic cloves, finely chopped (1 tablespoon)
• 3 tablespoons all-purpose flour
• 1 teaspoon dry mustard
• ½ teaspoon cayenne pepper
• ¼ teaspoon ground nutmeg
• 2 cups whole milk
• 4 ounces sharp white cheddar cheese, shredded (about 1 cup)
• 6 ounces Monterey Jack cheese, shredded (about 1 1/2 cups), divided

Direction

1. Fill a large bowl with ice water; set aside. Bring 12 cups water and 2 tablespoons salt to a boil in a large stockpot over high. Add spinach; cook, stirring constantly, until wilted, about 1 minute. Using a slotted spoon or a spider, transfer spinach to prepared ice water; let cool 1 minute. Drain well; place spinach on a clean kitchen towel, and squeeze firmly over sink to remove as much liquid as possible. Place spinach in a food processor; pulse until roughly chopped, 4 to 6 pulses. Set aside.
2. Preheat oven to 350°F with rack about 10 inches from upper oven heat source. Meanwhile, melt butter in a 10-inch cast-iron skillet over medium-high. Add onion; cook, stirring occasionally, until tender, about 4 minutes. Add garlic; cook, stirring constantly, until fragrant, about 30 seconds. Reduce heat to medium. Stir in flour, mustard, cayenne, nutmeg, and remaining 1 teaspoon salt; cook, stirring constantly, 1 minute. Gradually whisk in milk until combined; bring mixture to a boil over medium, whisking occasionally. Boil, whisking occasionally, until thickened, about 2 minutes. Stir in cheddar and 1/2 cup of the Monterey Jack cheese; cook, stirring constantly, until smooth, about 1 minute. Remove from heat; stir in reserved spinach until combined. Sprinkle evenly with remaining 1 cup Monterey Jack cheese.
3. Bake in preheated oven until bubbly, about 15 minutes. Increase oven temperature to broil (do not remove skillet from oven). Broil until browned, 2 to 5 minutes. Let stand at room temperature 5 minutes before serving.