Heat 2 tbsp olive oil in a large skillet over medium-high heat. Sear chicken until golden, 3 minutes per side. Transfer to plate.
Add the remaining 1 tbsp olive oil and 2 tbsp butter. Sauté mushrooms for 5–6 minutes until browned. Stir in garlic; cook for 1 minute.
Pour in Marsala wine, scraping browned bits; reduce by half.
Add chicken stock (and cream, if using). Simmer for 5 minutes until slightly thickened.
Return chicken to skillet. Simmer 5–8 minutes until cooked through and sauce coats the spoon.
Garnish with parsley. Keep warm while making orzo.
Heat 1 tbsp olive oil in a medium saucepan. Add onion; cook 3 minutes until softened. Stir in garlic and orzo, season lightly with salt and pepper, cook 1 minute.
Pour in broth and milk. Bring to boil, reduce heat, cover, and simmer 10 minutes, stirring occasionally, until orzo is just tender and liquid mostly absorbed. (Add more broth if dry before pasta is done.)
Stir in peas, spinach, and Pecorino. Cook 1–2 minutes until peas are hot, spinach wilted, and cheese melted.
Taste and adjust seasoning with salt and pepper.