One after the other pulse first the mushrooms until finely chopped, remove into a small bowl, wipe the food processor clean and then pulse the cauliflower. Depending on your blender I would recommend working in smaller batches until its finely chopped and the size of a rice grain. Put aside into a medium bowl.
Heat 1/4 cup of oil plus 2 tbsp of butter in a large pot. Over medium high heat, ddd mushrooms and cook until golden brown, stirring occasionally for about 4-6 minutes. Add the finely chopped onion and 2 tbsp oil. Cook until onion is very soft and golden, 6-8 minutes. Add garlic, dried tomato flakes, italian herbs and rosemary, cook until garlic is softened and fragrant about 2-3 minutes. Add tomato paste and cook until slightly darkened, about 2 minutes. Add the tomato sugo and the crushed tomatoes and cook until it thickens, 2-3 minutes. Add cauliflower and cook until it's slightly cooked down and slightly sticking to the pot, about 6-8 minutes. Season with salt and set aside.
Now cook pasta, following the package directions, until almost al dente, 1 minute short. Save 250ml of pasta cooking liquid.
Transfer pasta to the pot with the sauce. Ad the grated parmesan, remaining 2 tbsp of melted butter and 200 ml of the pasta cooking liquid. Cook on medium high heat, stirring until everything is good combined, until pasta is al dente and sauce is clinging to the pasta. about 3 minutes. If needed season with salt and pepper.