Bring a large pot of salted water to a boil. Cook the rotini pasta al dente, according to package directions. Reserve one cup of pasta water, then drain the rest. Set the drained pasta and pasta water aside.
To a large skillet over medium heat, add oil. Once hot, add sausage and cook for about 3-5 minutes on each side, or until brown. Transfer to a plate and tent to keep warm.
To the same skillet, add butter. Once melted, add onions. Cook until softened (about 3-5 minutes).
Add garlic and sundried tomatoes. Cook for one more minute.
Stir in the tomato paste, Italian seasoning, salt, and pepper.
Pour in the heavy whipping cream and stir until well combined.
Add the fresh spinach to the skillet, stirring just until it wilts, about 1-2 minutes.
Return the browned andouille to the skillet and add the drained pasta. Stir to coat the pasta in the sauce. If the sauce is too thick, add a bit of the reserved pasta water, a tablespoon at a time, until you reach the desired consistency.
Remove the skillet from heat. Stir in the parmesan cheese until melted.
Serve warm.