Bring a large pot of salted water to a boil and cook the pasta until almost al dente. Reserve some pasta water, then drain.
Heat olive oil in a wide pan over medium heat. Add the sausage with a sprig of rosemary, breaking it up gently and letting it brown properly to develop flavor.
Season lightly with salt and pepper, then add the chopped onion. Cook until soft and slightly sweet, allowing it to blend into the base.
Add the crushed tomatoes and cherry tomatoes, stirring gently and letting them soften and break down while keeping some texture.
Pour in a small splash of stock to loosen the sauce, then let it simmer gently for 15–20 minutes until slightly reduced and balanced.
Add the drained pasta directly to the sauce and toss over low heat so it absorbs the flavors and finishes cooking.
Stir in grated parmesan and a pinch of dried oregano. Adjust the consistency with a little reserved pasta water if needed, then serve immediately.