Place washed spinach in a large microwave-safe bowl and microwave for 5 minutes.
Let spinach cool and squeeze out as much water as possible. Best to use a cheese cloth and tongs to not burn hands. Once water is drained roughly chop spinach.
In a large-sauté pan over medium heat, melt butter. Add in chopped onions, salt, nutmeg, and dried thyme. Cook onions for about 2 minutes until they become soft.
Preheat oven to 350℉.
Stir in flour to form a roux. Let roux cook for about 1 minute until lumps are no longer visible.
While stirring, slowly add in milk. Continue to occasionally stir and allow mixture to come to a gentle boil for it to thicken.
Turn off the heat and pour in shredded Parmesan and ¾ of the grated Parmesan while continually stirring.
Stir in chopped spinach and mix thoroughly. Pour the mixture into a large casserole dish and top with remaining grated Parmesan. You can add more shredded Parmesan if you’d like!
Sprinkle top with a light amount of extra nutmeg and bake for 10-15 minutes.
Remove from oven and enjoy!
