Cook the sausages in a frying pan over medium-high heat until they register at 160°F. Remove and set aside.
In the same frying pan, add the onions and cook for 2 minutes. Add the butter and cook until the onions just begin to brown, about 8 minutes. Add the flour and stir together. It will be clumpy.
Add the mustard, worcestershire sauce, beer, and beef broth.
Bring to a boil and the reduce the heat. Simmer for 10 minutes. Add the sausage and simmer until the sausage is warmed again.
When serving, place a portion of the mashed potatoes in a bowl, top with sausage and gravy, and add a small scoop of warm peas.