Heat olive oil and 1 tablespoon of butter in a large, deep skillet or Dutch oven over medium-high heat. Season chicken pieces with salt, pepper, and Italian seasoning. Sear for 5 to 6 minutes, flipping once or twice, until golden brown and cooked through. Transfer to a plate and set aside.

Reduce heat to medium and add remaining 2 tablespoons of butter. Add sliced mushrooms and diced onion. Cook for 5 to 8 minutes, stirring occasionally, until mushrooms are browned and onions are soft and translucent. Add minced garlic and cook for 1 minute until fragrant.

Add dry orzo pasta to the skillet. Stir constantly for about 2 minutes to lightly toast the pasta until slightly golden and aromatic.

Pour in Marsala wine, using a wooden spoon to scrape up browned bits from the bottom. Let simmer for about 2 minutes until reduced by roughly half.

Add 3.5 cups of chicken broth and stir to combine. Bring to a boil, then reduce heat to medium-low to maintain a gentle simmer. Cook uncovered for 10 to 12 minutes, stirring every 2 to 3 minutes, until orzo is tender and most liquid is absorbed. Add remaining broth if needed.

Stir in heavy cream or Greek yogurt and grated Parmesan cheese. Mix until cheese melts and sauce becomes smooth and velvety.

Add baby spinach and seared chicken with any accumulated juices. Stir for 1 to 2 minutes until spinach wilts and chicken is heated through. Taste and adjust seasoning with additional salt and pepper if needed.

Remove from heat and garnish with freshly chopped parsley and extra Parmesan. Serve immediately while hot and creamy.