The sausage needs very little attention, the potatoes take the longest, and the gravy is the most active. So start with (1) the potatoes, then (2) the sausage, then (3) the gravy.
Heat the butter in in a medium skillet over medium high heat. Add the sausage and beer and cover for 6-8 minutes.
Turn the sausage and cover for another 6-8 minutes. Turn the temperature to low and let sit until the gravy and mashed potatoes are done.
Place the potatoes chunks in a saucepan and cover them with 2 inches of water and some salt. Boil them until a fork can easily slide into the potato pieces. About 15-20 minutes. Drain the water.
Add the half-&-half, butter, garlic powder, salt, and pepper. Using a potato masher or stand up mixer, mash the potatoes.
Heat the oil and 1 tablespoon butter in a medium skillet over medium-high heat.. Add the onions and mushrooms. Season with salt and pepper. Cook until soft, about 7-8 minutes.
Lower the heat to medium and add the flour and garlic powder. Mix well.
Pour in the beef stock, beer, and worcestershire sauce. Season with salt. Let the gravy simmer for about 5 minutes stirring often to prevent the flour from sticking to the bottom. Add the remaining butter and taste. Adjust the salt if needed.
Heat the peas according to the instructions on the package.