1. Prepare and Season the Chicken:
* Pat the cubed chicken dry with paper towels. This helps it sear better.
* In a medium bowl, toss the chicken pieces with the 1.5-2 tablespoons of Cajun seasoning (or your custom spice blend). Ensure each piece is well-coated. Set aside.

2. Cook the Pasta:
* Bring a large pot of generously salted water to a rolling boil.
* Add the pasta and cook according to package directions until al dente (tender but still with a slight bite).
* Crucially, before draining, reserve about 1-1.5 cups of the starchy pasta water. This water is liquid gold for creating a smooth, emulsified sauce.
* Drain the pasta and set aside. You can toss it with a tiny drizzle of olive oil to prevent sticking if it will sit for more than a few minutes.

3. Cook the Chicken:
* While the pasta is cooking, heat 1 tablespoon of olive oil in a large, deep skillet or Dutch oven over medium-high heat.
* Once the oil is shimmering, add the seasoned chicken to the skillet in a single layer. Do not overcrowd the pan; cook in batches if necessary to ensure proper searing.
* Cook for 3-5 minutes per side, until golden brown and cooked through.
* Remove the cooked chicken from the skillet and set it aside on a plate.

4. Sauté Aromatics and Build Sauce Base:
* To the same skillet (do not wipe it out – those browned bits are flavor!), add the remaining 1 tablespoon of olive oil and 2 tablespoons of butter. Melt over medium heat.
* Add the chopped yellow onion and cook for 4-5 minutes, stirring occasionally, until softened and translucent.
* Add the minced garlic and 1/2 – 1 teaspoon of red pepper flakes. Cook for another 1-2 minutes, stirring constantly, until fragrant. Be careful not to burn the garlic.
* If using tomato paste, add it now and cook for 1-2 minutes, stirring, until it darkens slightly. This deepens its flavor.
* If using flour for thickening, sprinkle it over the aromatics and cook, stirring constantly, for 1 minute.

5. Deglaze and Add Liquids:
* Pour in the 1/2 cup of chicken broth, scraping up any browned bits (fond) from the bottom of the skillet with a wooden spoon. This adds immense flavor. Bring to a simmer.
* If using, stir in the can of diced tomatoes (undrained or drained).
* Reduce the heat to medium-low.

6. Create the Creamy Sauce:
* Slowly pour in the 1 cup of heavy cream or half-and-half, stirring constantly.
* Bring the sauce to a gentle simmer (do not boil rapidly).
* Stir in the 1/2 cup of freshly grated Parmesan cheese until melted and the sauce is smooth.
* Season the sauce with salt and freshly ground black pepper to taste. Remember that the pasta water and Cajun seasoning on the chicken also contain salt.

7. Combine Everything:
* Return the cooked chicken (and any accumulated juices from the plate) to the skillet with the sauce. Stir to combine and allow the chicken to heat through, about 1-2 minutes.
* Add the cooked pasta to the skillet. Toss gently but thoroughly to coat all the pasta with the spicy creamy sauce.
* If the sauce seems too thick, add a splash of the reserved pasta water, a few tablespoons at a time, until it reaches your desired consistency. The sauce should be glossy and cling to the pasta. Simmer for another 1-2 minutes for the pasta to absorb some of the sauce.
* If using, stir in 1-2 teaspoons of fresh lemon juice to brighten the flavors.

8. Serve and Garnish:
* Taste one last time and adjust seasonings if needed (more salt, pepper, or red pepper flakes).
* Ladle the Spicy Chicken Pasta into warm bowls.
* Garnish generously with additional freshly grated Parmesan cheese, chopped fresh parsley or basil, and a pinch of extra red pepper flakes if desired.
* Serve immediately and enjoy the explosion of flavors!