Preheat the oven to 425 degrees F (220 degrees C). Coat a 16 inch pizza pan with nonstick cooking spray.
Place sausage in a skillet over medium-high heat. Cook, stirring to crumble, until evenly browned. Drain grease, and set aside.
In a large bowl, toss together the hash browns, salt, and 3/4 cup of the cheese until evenly blended. Spread in an even layer on the pizza pan, and pack down. Slice tomatoes thinly, and set on paper towels to drain. This will keep them from being watery when they cook.
Bake for 30 to 35 minutes in the preheated oven, until the crust is nicely browned on the top and bottom. Remove from the oven, and arrange tomato slices all over the pizza. Sprinkle the remaining cheese, mushrooms and sausage evenly over the pizza. Return to the hot oven and bake for another 5 to 10 minutes, until cheese is melted. Cool for at least 5 minutes before cutting into slices.