In a large pot, bring salted water to a rolling boil over high heat. Add the penne pasta and cook according to package instructions until al dente, typically 8-10 minutes. Once cooked, drain the pasta, reserving 1/4 cup of the starchy pasta water.

While the pasta cooks, season the peeled and deveined shrimp with salt, black pepper, and Italian seasoning. In a large skillet over medium heat, melt 2 tablespoons of unsalted butter. Add the shrimp and sauté for about 4-6 minutes, or until they turn pink and opaque. Remove from heat and tent with foil to keep warm.

Using the same skillet, add the remaining 2 tablespoons of butter over medium heat. Once melted, add finely chopped onion and sauté for about 4-5 minutes until softened and translucent. Stir in minced garlic and crushed red pepper flakes, cooking until fragrant for about 30-60 seconds.

Sprinkle the all-purpose flour over the onion and garlic mixture, stirring continuously for 1-2 minutes. Gradually whisk in the chicken or vegetable broth, ensuring no lumps form. Then, mix in the sun-dried tomatoes, tomato paste, Italian seasoning, salt, and black pepper.

Lower the heat and whisk in the heavy whipping cream and whole milk, allowing the mixture to simmer gently. Add freshly grated Parmesan cheese, stirring until melted and creamy. Toss in the fresh spinach, cooking for an additional 2-3 minutes until wilted.

Toss the cooked pasta along with the reserved pasta water into the creamy sauce, stirring to evenly coat. Gently fold in the sautéed shrimp, ensuring everything is heated through. Serve garnished with additional Parmesan cheese and minced fresh basil.