Bring a large pot of salted water to a boil. Cook linguine according to package instructions until al dente. Drain, reserving 1/2 cup of pasta water. Set aside.

In a large skillet over medium-low heat, add 2 tablespoons of olive oil or butter. Season the chicken strips with salt and pepper. Add the chicken to the skillet and cook until slightly golden on all sides. Sprinkle in the crushed red pepper flakes, paprika, and Italian seasoning. Continue cooking until the chicken is fully cooked through, about 5 minutes. Remove chicken from skillet and set aside.

In the same skillet, add a bit more butter or oil if necessary. Sauté the chopped onion and minced garlic for about 1 minute until fragrant. Add the chopped tomatoes and cook for another 2 minutes. Pour in the reserved pasta water and chicken stock. Bring to a simmer and reduce the sauce for about 3 minutes.

Add the fresh spinach to the skillet and stir until wilted. Season the sauce as needed. Add the cooked pasta and chicken back into the skillet. Toss everything together until well combined and heated through.

Adjust seasoning with more salt and pepper if necessary. Top with grated cheese if desired. Serve immediately and enjoy!