Place spinach in a large microwave-safe bowl, cover with damp paper towel, and microwave 1-2 minutes until wilted.
Chop spinach and squeeze out excess water.
Heat olive oil in a large skillet over medium heat and cook diced onion 5 minutes until translucent.
Add garlic and cook 2 minutes, stirring constantly.
Stir in garlic powder, onion powder, salt, and black pepper.
Add butter and let melt, then sprinkle in flour.
Cook 3-5 minutes, stirring constantly, until mixture smells nutty.
Slowly pour in heavy cream and milk while stirring constantly.
Add cream cheese, red pepper flakes, and nutmeg, stirring until cream cheese melts.
Reduce heat to low and simmer 3-5 minutes until sauce thickens.
Stir in 1/2 cup Parmesan, lemon juice, and chopped spinach.
Remove from heat and transfer to oven-safe dish.
Sprinkle remaining Parmesan on top and broil 2-3 minutes until golden brown.