Add the turkey stock and thyme to a saucepan over medium-low heat and bring to a simmer.
Add the butter to a straight-sided saute pan and let melt. Add the flour and cook over medium-high to high heat, whisking frequently, until the mixture is light brown in color, 5 to 10 minutes (for a dark gravy, cook the flour to a deeper brown color). Add 2 cups of the warm stock, whisking constantly, and cook until the gravy thickens and bubbles. Add more warm stock if a thinner consistency is desired. Reduce the heat to low and add salt and pepper to taste (this will depend on how salty the stock is). Add a squeeze of lemon juice and taste again for seasoning. Keep warm until ready to serve, adding more warm stock as needed.