Melt butter in 12-inch cast-iron skillet over medium heat. Add spinach, artichokes, and onion. Sauté, stirring frequently, for 5 minutes until spinach wilts and onions turn translucent.
Add garlic, salt, black pepper, cayenne pepper (optional), sage, and nutmeg. Sauté for 1 minute until garlic becomes fragrant.
Stir in cauliflower rice, broth, and lemon juice. Bring to rolling boil and continue boiling for 10 minutes, stirring frequently.
Add cubed cream cheese and stir until fully melted and combined into smooth sauce.
Sprinkle Parmesan cheese evenly across top of casserole.
Transfer skillet to oven and broil for 5 minutes until golden brown and edges are bubbling.
Remove from oven and rest for 5 minutes before serving.
Notes
Use canned artichoke hearts, not marinated varieties.
Can substitute olive oil for butter.
Can use 4 ounces frozen spinach (thawed and squeezed dry).
To make vegetarian: use vegetable broth and substitute mozzarella for Parmesan.
Soften cream cheese to room temperature for easier melting.
Use freshly shredded Parmesan for best melting quality.
Rice cauliflower in food processor with shredding disc for best texture.
Can use pre-riced frozen cauliflower (fully thawed and squeezed dry first).