Pat the chicken thigh pieces dry with paper towels — this is not optional, moisture on the surface creates steam instead of a sear and you'll end up with grey, sad chicken instead of golden brown pieces. Season all over with smoked paprika, garlic powder, onion powder, Italian seasoning, half the salt, the black pepper, and cayenne if using. Toss to coat evenly.
Heat the olive oil in a large skillet or wide Dutch oven over medium-high heat until shimmering. Add the chicken pieces in a single layer — do not crowd them, work in two batches if needed. Sear for 3 to 4 minutes without moving until deeply golden on the bottom, then flip and cook another 2 minutes. The chicken does not need to be fully cooked through at this point. Remove to a plate and set aside.
Reduce heat to medium. Add the butter to the same pan — do not wipe it out, those browned bits on the bottom are flavor. Once the butter is melted, add the minced garlic and cook for 60 seconds, stirring constantly, until fragrant.
Add the uncooked rice to the pan and stir to coat it in the butter and garlic. Toast for 1 minute, stirring constantly — the rice will pick up the fond from the chicken sear and turn slightly translucent at the edges. Pour in the chicken broth and remaining salt. Scrape the bottom of the pan to release any stuck bits. Bring to a boil, then reduce heat to low, cover tightly, and cook for 15 minutes without lifting the lid.
After 15 minutes, check the rice — it should be just about tender with most of the liquid absorbed. Nestle the seared chicken pieces back into the rice, cover again, and cook on low for 5 more minutes until the chicken is fully cooked through.
Remove the lid. Pour the heavy cream over everything and stir gently to combine. Add the shredded cheddar and grated parmesan a handful at a time, stirring between each addition on low heat until the cheese is fully melted and the sauce is smooth and glossy. Taste and adjust salt.
Let the pan rest off heat for 3 minutes — the sauce tightens slightly and the rice finishes absorbing. Garnish with sliced green onion and fresh parsley and serve directly from the pan.