Bring a large pot of salted water to a boil. Add the linguine and cook until al dente (about 9-10 minutes). Drain and set aside.
Slice each chicken breast in half horizontally to create 4 smaller chicken breasts. In a small bowl, combine paprika, cayenne pepper, kosher salt, and black pepper. Season the chicken breasts evenly on both sides with the seasoning mixture.
To a large skillet over medium-high heat, add the olive oil. When hot, add the chicken breasts and cook for 2-3 minutes on each side, or until they turn golden brown and reach an internal temperature of 165°F. Remove the chicken from the pan and tent it with foil to keep it warm.
In the same skillet, melt the butter. Add the diced onion, sun-dried tomatoes, and tomato paste. Cook for 2-3 minutes, or until the onion has softened. Add the minced garlic and cook for 1 more minute.
Stir in the chicken stock to deglaze the pan slightly, scraping the bottom bits as you go. Pour in the heavy cream, parmesan cheese, crushed red pepper flakes, kosher salt, and black pepper. Reduce the heat to medium-low and bring the sauce to a simmer.
Add spinach. Cook until wilted, about 3-4 minutes.
Add the cooked linguine to the sauce and toss to combine.
Top the pasta with the chicken breasts. Serve immediately.