Melt 3 tablespoon butter in a medium saucepan over medium-high heat.
When butter melts, turn heat to medium and add sliced onions and cook until they are softened and turning golden brown, about 8-10 minutes.
Sprinkle 3 tablespoon flour over cooked onions and cook for 2-3 minutes (on medium heat) more to cook off the raw flour taste. Stir the mixture often to keep it from burning and ensuring it is well mixed.
Add ½ cup beer to onion mixture, stirring up any caramelized bits on the bottom of the pan. Stir until well combined. Then pour in the 1.5 cups beef broth and simmer onion gravy for 10 to 15 minutes allowing it to thicken and develop and cook off the alcohol.
Notes
If you don't like or want to cook with beer, just replace it with more beef broth for a delicious savory gravy.
Use vegetable broth instead of beef broth for a vegetarian version.
Add a spring of fresh thyme or bay leaves.
Try a couple teaspoons of brown sugar to add a touch of sweetness.
Try a teaspoon or two of Worcestershire sauce for added umami flavor.
If you have made the gravy and find the beer taste too strong, try adding brown sugar, Worcestershire sauce and a splash of heavy cream.