Bring a large pot of salted water to a rolling boil over high heat. Once boiling, add penne pasta and cook according to package directions until al dente, about 10–12 minutes. Reserve 1/4 cup of pasta water for later use, then drain the pasta and set it aside.

While the pasta cooks, season shrimp with 2 teaspoons of Italian seasoning, along with salt and pepper to taste. In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Add the shrimp and cook for about 4 to 6 minutes, turning occasionally until they are opaque and pink. Remove the shrimp from the skillet and tent with foil to keep warm.

In the same skillet, melt the remaining 2 tablespoons of butter over medium heat. Add a small finely chopped onion and sauté for about 4 to 5 minutes until softened and translucent. Stir in minced garlic and crushed red pepper flakes, cooking for an additional 30 to 60 seconds until fragrant.

Sprinkle flour over the sautéed mixture, continuously stirring for about 1 to 2 minutes until lightly browned. Gradually whisk in broth, ensuring there are no lumps, and then add sun-dried tomatoes and tomato paste. Reduce the heat to low and simmer.

Whisk in heavy whipping cream and whole milk, stirring until well mixed. Add grated Parmesan cheese, continuing to stir until fully melted and incorporated. Toss in spinach and cook for 2 to 3 minutes until wilted.

Gently stir in the cooked penne pasta along with reserved pasta water. Add the cooked shrimp back into the skillet, warming through for about 1 to 2 minutes.

Spoon onto plates or a serving dish. Garnish with additional Parmesan cheese and fresh basil leaves before serving.