Bring water to a boil and cook your pasta according to the package directions. Once ready, reserve 1 cup of pasta water, drain, and set the pasta aside.
Prep the chicken:
On a flat surface, pat dry the chicken breasts. Butterfly the chicken breasts by slicing them horizontally through the middle.
Season both sides of the chicken breasts generously with the blackened seasoning.
Cook the chicken:
Heat a skillet over medium-high heat. Once hot, melt 1 tablespoon of butter. Place the chicken breasts on the pan, and let them cook for 3 to 4 minutes. Flip the chicken breasts over, and cook for another 4 minutes on the other side. *If using whole chicken breasts, the cooking time will vary to 7 to 8 minutes per side until the internal temperature has reached 165F.
Remove the chicken from the pan onto a plate and leave any tidbits on the pan for extra flavor when making the sauce next.
Make the dish:
Add the remaining tablespoon of butter to the pan and melt it while scraping the tidbits with a spatula. Add the chopped onion and sauté for 3 to 5 minutes, until soft and fragrant. Add the garlic, diced tomatoes, and sauté for 2 minutes.
Add the chicken stock and tomato paste and combine to mix the tomato paste into the thick mixture. Add ½ cup of reserved pasta water and the parmesan cheese and combine into the sauce.
Add the cooked pasta and stir to combine with the sauce. If it’s too thick, slowly add additional reserved pasta water to thin it out to a consistency of your liking.
Slice the chicken breasts and toss them into the pasta.
Sprinkle parsley over the top (if using) and additional parmesan cheese when serving.
Notes
*Substitute fire-roasted tomatoes for regular or Italian-seasoned diced tomatoes.