In a small pot melt the butter on medium heat and add the crushed garlic. Reduce to low heat and cook until the garlic becomes fragrant but not brown, about 3-5 minutes.
Add the cream and milk and stir with a whisk until mixture starts to blend comes to a simmer, about 10 minutes.
Add aromat and fish spice and continue to stir the mixture on low heat until it starts to thicken gradually. This will take up to 10 minutes.
Lastly add fresh parsley and keep stirring to prevent burning.
Remove from the heat and whisk in fresh lemon juice, then use immediately.
If using with mussels, add the cooked mussels to the sauce and allow to simmer for 5 - 10 mins before removing from the stove.
Alternatively,the sauce can be added to the cooked mussels afterward.
Notes
This sauce is enough for 500 grams of black mussels or fish.