Cook the pasta. Bring a large pot of salted water to a boil. Add 12 ounces of rotini pasta and cook one minute less than the package instructions (it will finish cooking in the sauce). Before draining, reserve 1-2 cups of the starchy pasta water. Drain the pasta and set it aside.
Soften the onion. Return the pot to medium heat. Melt the butter, then add the onion. Sauté for 4-5 minutes, stirring until softened and translucent.
Sauté the sun-dried tomatoes and seasoning. Stir in the sun-dried tomatoes and cook for 1 minute, releasing their flavor into the butter. Add the minced garlic, dried oregano, and salt, stirring for 30 seconds until fragrant (be careful not to burn the garlic).
Make the creamy sauce. Pour in two cups of room-temperature heavy cream and two cups of low-sodium chicken broth (for a more decadent sauce, reduce the chicken broth to one cup), stirring to combine. Bring the mixture to a gentle simmer (not boiling!) over medium-low heat, allowing the flavors to meld. Simmer for 3–4 minutes, stirring continuously, until the sauce thickens slightly.
Add the spinach and pasta. Stir in the chopped spinach, letting it wilt for 1–2 minutes, then add the cooked pasta. Gently toss to coat. If the sauce is too thick, add a splash of reserved pasta water (1–2 tablespoons at a time) until you reach your desired consistency.
Season and serve. Taste the sauce and adjust the seasoning if needed. Remove from heat and stir in freshly grated parmesan cheese, if using, and garnish with fresh parsley, oregano, or basil, if desired.