Add the butter and garbanzo bean flour to the pan and stir over medium heat until it resembles wet sand.
Gradually add the stock and milk while whisking.
Simmer the gravy until thickened.
Add onions, mustard, red pepper flakes, salt, and pepper.
Taste for seasoning adjustments and serve!
Notes
I sweat onions in bulk and keep them in the freezer. If you don't have that on hand, you'll need to sweat your onions before the first step in the gravy recipe.
Recipe by Creating Silver Linings at https://www.creatingsilverlinings.com/mustard-gravy/