2tablespoonsoil or fat of choice- I used a combination of butter and bacon fatbut if you want to lighten it up a bit a neutral oil or even olive oil would work too
16ouncesof Guinnessif you want a creamier gravy I would go with 8-12 ounces of beer and 8 ounces of milk- or do like I did and use half the gravy for dinner and then add a cup of milk the next morning for breakfast
Worcestershire to taste- start with about a teaspoon and go up just a drop or two at time from there
chives or parsley for garnish
in a dry preheated cast iron (or other heavy bottomed) pan over medium low heat add the mushrooms and onions and cook until the mushroom begin to sweat and the onions begin to brown
once the mushrooms have given off most of their moisture add the mustard, butter and bacon grease, and flour. Cook over medium heat stirring constantly until the flour is completely saturated with the oils, no dry bits of flour should remain.
Reduce the heat to medium low and slowly add about four ounces of the Guinness, whisking to combine into a smooth paste (except, of course, for the onions and mushrooms).
Once that is incorporated continue to add the liquid. Heat, stirring frequently, until the gravy is just bubbly and thickened to your taste. If you are just doing beer you can let this sit on low for an hour or two, it will only make the veggies more tender and the gravy richer.
Guinness and Mushroom Gravy https://www.recipefiction.com/2018/03/guinness-and-mushroom-gravy.html