Add the Italian sausage to a Dutch oven or large stockpot over medium-low heat. Cook, breaking the sausage into crumbles as it browns, until completely cooked through. Transfer to a paper towel-lined plate. Remove the excess grease from the pot, leaving about 1 tablespoon behind.
Add the diced onion to the reserved sausage drippings and cook for about 5 minutes, until softened and translucent. Add the garlic and cook for about 1 minute, just until fragrant
Stir in the Italian seasoning, black pepper and red pepper flakes. Add the tomato paste and stir until evenly incorporated, then add the drained, chopped sun-dried tomatoes. Cook for another 1–2 minutes, stirring occasionally, until the tomato paste deepens slightly in color.
Sprinkle the flour evenly over the onion and tomato mixture. Stir continuously for about 1 minute, until the flour is completely incorporated and no dry streaks remain.
Slowly pour in the white wine while whisking or stirring continuously, scraping up any flavorful browned bits from the bottom of the pot. Cook for 2–3 minutes, allowing the wine to reduce slightly and its sharp alcohol aroma to mellow.
Gradually pour in the chicken stock while stirring, followed by the heavy cream. Return the cooked sausage to the pot and stir to combine.
Bring the soup to a boil, then add the gnocchi. Cook for about 4–5 minutes, stirring occasionally to prevent the gnocchi from sticking to the bottom of the pot, until plump and tender. Avoid overcooking, which can make the gnocchi gummy.
Reduce the heat to a gentle simmer. Add the baby spinach and stir just until wilted. Add the freshly grated Parmesan and stir until completely melted into the broth.
Taste the soup and add salt only if needed. Ladle into bowls and serve warm with additional freshly grated Parmesan and crusty toasted bread or buttery breadsticks for dipping.