8 Russett potatoes, peeled and quartered
2 tablespoons unsalted butter
1/4 to 1/2 cup whole milk
Salt and pepper to taste
1 1/2 pounds beef sausage
1/2 cup diced onion
1 teaspoon rosemary
1 teaspoon oregano
1 garlic clove, minced
1/4 cup mushrooms, sliced
1 (.75 oz) packet dry brown gravy mix
1 cup water
Preheat oven to 350F. Place potatoes in a saucepan with just enough water to cover. Bring to a boil, then lower heat and cook potatoes until tender, about 25 minutes. Drain, then mash with 4 teaspoons of the butter and enough milk to reach a creamy consistency. Continue mashing until smooth. Season to taste with salt and pepper.
In a large skillet over medium heat, cook sausage until heated through, about 3-5 minutes. Remove sausage from skillet and set aside. Add remaining butter to skillet, and add onion, rosemary, oregano, garlic, and mushrooms. Stir until tender. Whisk together gravy mix and water as directed on packet, then add to skillet with onion and mushrooms. Simmer, stirring constantly, to form a thick gravy.
Pour half of the gravy mix into a medium sized casserole dish so that the bottom is coated. Place sausages in a layer over the gravy, then cover with remaining gravy. Top with mashed potatoes.
Bake bangers and mash uncovered in preheated oven for 25 minutes or until potatoes are evenly browned.