If using fresh spinach, bring a large saucepan filled two-thirds full with water to a rolling boil. Add the spinach and cook for 1 to 2 minutes, or just until spinach wilts and water returns to a boil. Remove from heat and strain spinach in a colander. Rinse under cold running water until cool.

Using your hands, squeeze spinach to remove as much liquid as possible. You should have about 1 ½ cups of spinach. Place spinach on a cutting board and chop finely; set aside.

In a heavy medium saucepan, melt butter over medium-high heat. Add onion and cook until soft, about 3 minutes. Add garlic and cook until fragrant, about 1 to 2 minutes. Do not allow garlic to brown. Add flour and stir to combine. Cook, stirring constantly, until mixture is a light-blond color, about 1 to 2 minutes. 

While whisking, slowly add heavy cream and milk until smooth. Continue to cook until it comes to a boil and thickens, about 2 minutes. Reduce heat to medium-low and simmer for 3 to 4 minutes. Add the Parmesan cheese, anise-flavored liqueur, lemon juice, Essence, and salt; stir until well combined.