Heat up olive oil in a frying pan over medium heat. Add the sausages. Cook until they have browned on the bottom, then flip. Remove the sausages from the pan when they are brown on all sides.
6 pork sausages, 1 teaspoon olive oil
Add sliced onions to the same pan. Sauté for a few minutes until the onions are soft.
2 cups sliced onion
Add Italian seasoning and tomato paste to the onions. Stir to combine, and sauté for another minute or until fragrant. Add the flour, and stir to coat the onions evenly.
1 tablespoon tomato paste, ½ teaspoon dried herbs, ¼ teaspoon salt, ¼ teaspoon black pepper, 2 tablespoons all-purpose flour
Gradually add beef broth to the pan. The flour will cause a thick sauce to form. Add the mustard. Allow the onion sauce to simmer for a few minutes.
½ cup beef broth
Add the sausages and onion gravy to a pie dish.
Meanwhile, peel and dice the potatoes. Add them to a large pot, and cover with water. Bring this to a boil, and allow the potatoes to simmer for a few minutes, or until they are soft enough to easily pierce with a fork.
2 pounds potatoes
Drain off excess water from the potatoes, and add milk, seasoning, butter and cheese. Use a potato masher to mash everything together. Add extra cheese or seasoning to taste if you wish.
2 tablespoons butter, ½ cup cheddar cheese, ½ cup milk, ¼ teaspoon nutmeg
Add the mashed potatoes on top of the sausages in the pie dish. Bake the pie in a preheated oven at 400 F / 200 C for about 15 minutes, or until the mash is slightly golden on top.