in a saute pan cook diced onions and minced garlic in butter for 2-3 minutes until softened.
Place the spinach on cutting board and rough chop into smaller even pieces then begin adding the spinach to the pan in batches. If using frozen spinach drain first then chop before adding to the pan.
Once the spinach is cooked drain off ALL excess liquid then add in heavy cream, nutmeg, salt and pepper. Simmer until the cream begins to thicken then stirin parmesan cheese.
After the parmesan cheese has melted into the sauce you can flod in the sour cream and lemon juice. Turn heat to low then begin stirring in shredded mozzarella stirring until fully incorporated.