How to Make It
Preheat oven to 450°F.
Shred mozzarella using the large holes of a box grater; refrigerate until ready to use. Shred potatoes using the large holes of a box grater. Place shredded potatoes on several layers of paper towels or a clean kitchen towel; squeeze over the sink to remove as much liquid as possible. Place potatoes in a large bowl. Add 1/2 teaspoon salt, 1/2 teaspoon pepper, and mozzarella; toss well to combine.
Heat a 10-inch cast-iron skillet over medium heat. Add 1 tablespoon oil to pan; swirl to coat. Add potato mixture to pan. Using a wooden spoon or a dry measuring cup, carefully press potato mixture firmly into bottom and up the sides of the pan. Cook over medium heat for 5 minutes. Place pan in oven. Bake at 450°F for 15 minutes. Remove pan from oven, and reduce oven temperature to 350°F.
Meanwhile, heat a large skillet over medium-high heat. Add remaining 1 tablespoon oil to pan; swirl to coat. Add onion to pan; sauté 5 minutes. Add artichoke hearts, remaining 1/2 teaspoon salt, and remaining 1/2 teaspoon pepper; sauté 3 minutes. Gradually add spinach, stirring until spinach wilts, about 2 minutes. Remove from heat.
Place spreadable cheese in a medium bowl. Gradually add milk, stirring with a whisk until well blended. Whisk in chives, mustard, and eggs.
Spoon spinach mixture into potato crust. Pour egg mixture evenly over spinach mixture. Bake at 350°F until center of egg mixture barely jiggles, 38 to 40 minutes. Remove pan from oven; let stand 10 minutes before serving.