In a medium saucepan, heat the broth, concentrate, Worcestershire sauce, onion, rosemary and pepper over medium-high heat. Bring to a boil, then reduce the heat to low and simmer for 15-20 minutes.
In a large saucepan, melt the butter over medium heat. Add the flour and cook, stirring constantly, for 2 to 3 minutes until the mixture is light golden brown.
Slowly pour in the beef broth ⅓ cup at a time through a strainer (to catch any loose rosemary pieces), whisking well after pouring. Once all the stock is incorporated, whisk until it begins to thicken, 3-4 minutes.
Stir in the browning sauce, if using.
Taste for seasoning and add in more if necessary.
Turn the heat down to low and let it simmer for 5 minutes, stirring occasionally.
If using immediately, keep warm to serve.
If making ahead, let cool, then pour into a mason jar and refrigerate until ready to use. Reheat in a saucepan on medium-low, stirring occasionally. Then keep warm until ready to serve.