Add the sausage and chopped onion to a large skillet over medium-high heat. Break the sausage apart with your spoon as it cooks, letting it brown for about 7 to 8 minutes. This step builds the savory foundation the whole dish depends on.
Stir in the minced garlic and cook it for about 30 seconds, just until it turns fragrant. Garlic burns fast, so keep an eye on it and stir constantly. This quick step adds a wonderful aromatic layer to the sauce.
Pour in the white wine and let it simmer for about a minute, scraping up any browned bits from the bottom of the pan. Those browned bits are packed with flavor, so don't skip this step. The wine also helps mellow out the richness of the sausage.
Stir in the sun-dried tomatoes, diced tomatoes, heavy cream, and uncooked gnocchi. Bring everything to a gentle bubble, then cover the pan and reduce the heat to medium. Let it cook for about 5 minutes so the gnocchi can start absorbing the sauce.
Uncover the pan and stir well, then let it cook uncovered for a few more minutes. You're looking for the gnocchi to turn soft and the sauce to thicken up nicely. If it seems thin, just give it another minute or two on the heat.
Fold in the fresh spinach and let it wilt right into the hot sauce, which only takes a minute or so. Add the parmesan cheese, torn basil, Italian seasoning, salt, and pepper, then stir everything together. Serve immediately while it's hot and creamy.