Start by bringing a large pot of salted water to a boil over high heat for the linguine. Drain the sun-dried tomatoes, reserving the oil for later use. Chop the tomatoes into bite-sized pieces, and have your other ingredients ready, including diced chicken and chopped onion.

In a large skillet, heat 2 tablespoons of the reserved sun-dried tomato oil over medium-high heat. Season the sliced boneless chicken breasts with salt and black pepper, and sauté them for about 6–8 minutes, or until they turn golden brown and are cooked through. Remove the chicken from the skillet and set it aside on a plate, leaving the remaining oil in the skillet for the sauce.

In the same skillet, add chopped onion and sauté for 3–4 minutes until it becomes translucent and fragrant. Stir in a pinch of salt, black pepper, and the tomato paste, followed by the minced garlic, lemon zest, and red pepper flakes. Cook this mixture for an additional 1–2 minutes, allowing the flavors to meld and become aromatic.

Whisk in 1.5 cups of chicken broth, bringing the mixture to a gentle simmer over medium heat. Let it simmer for about 10 minutes, allowing the sauce to reduce slightly and thicken. Stir occasionally until it reaches a rich consistency, perfect for coating your pasta.

While your sauce simmers, cook the linguine in the boiling salted water according to package instructions until al dente, usually taking about 8–10 minutes. Just before draining, reserve about 1.5 cups of the starchy pasta water. Drain the linguine and set it aside, keeping it warm.

Reduce the heat to low and stir in the cubed cream cheese, fresh spinach, and the chopped sun-dried tomatoes into the sauce. Stir continuously until the cream cheese has melted and the sauce becomes creamy and smooth, about 3–5 minutes. If the sauce seems thick, gradually add some reserved pasta water.

Finally, add the cooked linguine and the sautéed chicken back into the skillet, tossing everything together gently to coat with the creamy sun-dried tomato sauce. Incorporate more reserved pasta water as needed to ensure a velvety finish. Serve immediately, garnished with additional grated Parmesan cheese for an extra flavor boost.