In a large Dutch oven over medium high heat, combine the canned tomatoes, butter onion halves and 1 teaspoon salt.

Stirring occasionally, simmer for an hour, breaking up the tomatoes into pieces with a wooden spoon as they cook and break down. After an hour, remove from heat and let cool.

Then remove the onion halves, storing for future use. Transfer the tomato mixture into a blender or food processor and blend until smooth.  Add salt to taste. Return the blended tomatoes to the Dutch oven. Set aside. (This can be done a day ahead.)

In a large sauté pan, over medium high heat, warm 1 Tbsp olive oil. 

Using your hands, break the sausage into nice size chunks and add to the pan. Browning evenly on all sides, cook until juicy and beginning to caramelize, about 5 minutes total.

Reserving the fat and oil in the pan, use a slotted spoon to remove the sausage and set aside.

If needed, add more oil to the pan. Add 4 cups of the sliced mushrooms and 1 tsp salt, cooking and stirring until brown, about 12 minutes. Add the garlic to the pan and cook for 2 more minutes. Remove and set aside. Add more oil if needed and repeat for the remaining 4 cups of mushrooms.

Add the sausage and mushrooms to the Dutch Oven and stir to combine.  Serve on top of the pasta and garnish with parmesan.