Heat beef stock in a medium / large pot on medium-high heat to boiling (about 10 minutes) to reduce by about ⅓ volume, for approximately an additional 30 minutes at a steady boil.
While the stock is reducing, in a small saucepan, melt butter on low heat, then add the flour and mix well. Let this mixture cook for about 2 minutes, whisking constantly.Note: make sure to keep the heat on low as roux can burn quickly!
Once the stock has reduced by 1/3 mix in the flour butter (roux) and bring to a boil to thicken.
Once boiling, reduce temperature to medium/ medium low to a continuous simmer to thicken the gravy (about 45-60 minutes).
Serve immediately with roasted turkey, pot roast, or mashed potatoes.
Notes
It is normal if some foam/bubbles form at the top of the gravy after adding the roux, it will stir in with the gravy as it thickens.