Preheat the oven to 400 degrees F (200 degrees C).
Melt unsalted butter in a saucepan over medium-low heat; stir in green onions and a pinch of salt.
Cook, stirring occasionally, until the onions are soft, about 5 minutes.
Stir in minced garlic into the onions and remove from heat.
Combine the green onion mixture with spinach, artichoke hearts, cream cheese, Gruyere, Parmigiano-Reggiano, hot sauce, nutmeg, salt, and pepper in a large bowl until well combined.
Spoon the artichoke mixture into two ramekins.
Top each with shredded mozzarella cheese.
Bake in the preheated oven until the tops are golden brown and bubbling, about 25 minutes.