Preheat oven to broil.
In a large cast-iron skillet, melt butter over medium-high heat to coat the bottom of the pan. Add onion and garlic and cook until soft, 3 to 5 minutes.
Season with salt, pepper, cayenne and nutmeg, and let cook another minute before adding milk. Simmer on medium-low until it begins to thicken, about 3 minutes.
Add kale, artichoke hearts, lemon zest and juice, and hot sauce to taste. Cook until warmed through, then melt in cream cheese, Greek yogurt, and parmesan. Cook until thick and creamy, taking care not to boil the mixture.
Sprinkle with mozzarella and broil until cheese is browned and bubbly.
Serve with warm sweet potato tortilla chips.