Prepare all ingredients by chopping the red onion, slicing the mushrooms, and chopping the garlic.

Cook gnocchi according to package instructions.

While gnocchi is cooking, heat olive oil in a frying pan over medium heat.

Add sausages to the pan and cook until browned and cooked through, breaking them up with a spoon.

Add red onion and chestnut mushrooms to the pan and cook until softened, about 5 minutes.

Add garlic and balsamic vinegar to the pan and cook for 1 minute.

Stir in chopped tomatoes and rosemary, and simmer for 10 minutes, or until the sauce has thickened slightly.

Drain the gnocchi and add it to the pan with the sausage and mushroom sauce.

Stir in basil leaves and serve immediately.

Sprinkle with parmesan cheese, if desired.