Pierce bangers and add to a heavy pot with olive oil and 5 sprigs of thyme and 3 sprigs of marjoram, bake in oven for 25-30 minutes
While the bangers are cooking, peel and cube potatoes and add to a 3 quart saucepan
Fill with enough water to cover potatoes
Season with kosher salt
Bring water to a boil and simmer for abut 10 minutes or until you can pierce easily with the tip of a knife. Do not overcook
Strain potatoes and return to pot. Add sour cream and butter
Mash with potato masher until the potatoes are smooth.
Peel the leaves from 3 sprigs of thyme and add to the pot. Add salt and pepper to taste. Add milk slowly, beating on low speed with an electric mixer until creamy. Set aside.
When bangers are cooked, remove pan from oven and set on the stovetop. Set the bangers aside on a plate
Add butter to the pan and melt over medium heat. Add the onions
Stir frequently to avoid the onions burning. You want them to mix with the pan juices and brown up nicely. Reduce heat to low and cook gently for about 10 minutes until tender. Season with kosher salt and pepper
Add garlic and mix with the onions. Sprinkle with half the flour and mix to incorporate
Add the Guinness and the beef broth
Stir until the sauce starts to thicken a bit. Dissolve the rest of the flour in the cold half and half. Add slowly to mixture to further thicken. Once the gravy is thickened to your liking, remove from heat.
To plate, start with the mashed potatoes. Top with one to two bangers and top with the gravy. I prefer serving with peas, but feel free to use the vegetable of your choice.