Place all the Creamy Sauce ingredients into a blender and blend until smooth. Taste and adjust the salt if needed.
In a large skillet, heat the sun-dried tomato oil over medium-high heat. Add the crushed garlic and sauté for about 20 seconds until fragrant. Add the diced onion and cook for 3–5 minutes, or until soft and translucent.
While the onion cooks, bring a large pot of salted water to a boil and cook the pasta until al dente—tender but still firm to the bite. Before draining, reserve about one mugful of the pasta water, then drain the pasta.
Add the drained pasta to the skillet with the onions and garlic. Pour in the blended Creamy Sauce, add all the shredded spinach, and about ½ cup of the reserved pasta water (see Note 5). Toss everything together gently to coat the pasta in the sauce and wilt the spinach. Let it cook for about 2 minutes until everything is heated through. Adjust seasoning if needed.
Use more of the reserved pasta water, a little at a time, to loosen the sauce to your desired consistency.
Remove from heat and serve immediately, topped with extra Parmesan and fresh basil, if using.