Toss the potatoes with the vegetable oil in a bowl until potatoes are well coated. Press the potatoes into a 10-inch pie pan to create a crust.
Bake potatoes in the preheated oven until golden brown, about 30 minutes. Remove pan from oven, and reduce heat to 350 degrees F (175 degrees C).
While the potatoes are baking, heat a large skillet over medium-high heat and stir in the sausage. Cook and stir until the sausage is crumbly, evenly browned, and no longer pink. Drain and discard any excess grease. Combine the sausage with the eggs, mushrooms, pepperjack cheese, Swiss cheese, milk, 1 tablespoon parsley, garlic salt, and black pepper in a large bowl. Pour the egg mixture into the prepared potato crust.
Bake in the oven until eggs are set, about 25 minutes. Arrange the tomato slices over the eggs and sprinkle the remaining 1 1/2 teaspoon parsley on top. Return to the oven and continue baking until the tomatoes soften, about 5 minutes more.