Pat chicken thighs dry and season both sides with 1 teaspoon kosher salt, ¼ teaspoon ground black pepper, and 1 teaspoon Italian seasoning.
Heat 2 tablespoons olive oil in a large cast iron skillet over medium-high heat until shimmering.
Add the chicken thighs and brown 3–4 minutes per side until golden; remove chicken and set aside.
Reduce heat to medium and add the remaining 2 tablespoons olive oil. Sauté the chopped ½ onion, sprinkling lightly with salt, until softened, about 2 minutes.
Add the minced garlic and chopped sun-dried tomatoes and cook 1 minute more until fragrant.
Stir in 1½ cups orzo and toast it with the aromatics for about 1–2 minutes.
Pour in 3 cups low-sodium chicken broth, bring to a boil, then reduce to a simmer.
Return the browned chicken thighs to the skillet, cover, and simmer for 10–15 minutes until the orzo is tender and the chicken is cooked through.
Remove the chicken to a plate. Stir 3 cups fresh spinach, ½ cup heavy cream, and ¼ cup grated Parmesan into the cooked orzo until the spinach wilts and the sauce is creamy.
Return the chicken to the skillet to warm through, finish with chopped fresh parsley, additional Parmesan if desired, and a squeeze of lemon juice before serving.