Melt butter in large pot over medium heat.
Add onion. Cook 3 to 4 minutes until soft.
Add garlic. Cook 30 seconds.
Add orzo. Stir to coat. Toast 1 minute.
Add broth. Stir well.
Bring to boil over high heat.
Reduce to medium-low. Cover with lid.
Simmer 10 to 12 minutes, stirring occasionally, until orzo is tender.
Remove lid. Stir in heavy cream.
Add spinach. Stir until wilted (1 minute).
Stir in Parmesan, salt, pepper, and nutmeg.
Add liquid if too thick.
Remove from heat. Stir in lemon zest if using.
Serve hot. Garnish with Parmesan, parsley, and lemon wedges.