Chef John's Mushroom Gravy

Ingredients:

1/4 C. butter
1 (16 oz.) package sliced mushrooms
salt to taste
1/4 C. all-purpose flour, or as needed
3-1/2 C. beef stock
1 pinch ground black pepper to taste
fresh thyme leaves, to taste (optional) I used a slight pinch of dried ground thyme

Directions:

Heat butter over medium heat in a saucepan until it foams.  Stir in mushrooms.  Season with salt.  Simmer until liquid evaporates, maybe about 20 minutes (could be less).
Stir in the flour, cooking and stirring for about 5 minutes.  Add about 1 cup of beef stock, stirring briskly until incorporated, then pour in the remaining stock and mix thoroughly.  Season with black pepper and thyme.  Reduce heat to medium-low and simmer until thickened, about 30 minutes, stirring often.