Heat milk and stock in saucepans or microwave.

Melt 1 Tbsp. butter until it stops frothing.

Add onion & celery and reduce heat.

Saute until vegetables are clear (5-10 min)

Add garlic and sweat for 1 minute.

Sift in 1 Tbsp. of flour and stir to make a roux.

Stir roux constantly until the flour is cooked, about 2 min.

Remove roux from heat and add stock and whisk until smooth.

Return to heat and add salt, pepper, thyme, and parsley.

Add milk and whisk until smooth.

Bring sauce to a boil, stirring continuously, then reduce heat to a simmer.

Allow sauce to simmer until reduced by half (or desired consistency, coating the back of a spoon).

Adjust seasoning to taste.