Heat oil in a large skillet set over medium heat. Add chicken and season with Italian seasoning, salt and pepper. Cook until golden and no longer pink, 8 minutes per side. Remove from the skillet and set aside.
Reduce heat to medium-low, then add butter and a splash of water, broth or white wine, deglazing the brown bits from the pan. Saute onions for 5 minutes until translucent, then stir in garlic and cook 30 to 60 seconds, until fragrant.
Add diced tomatoes and spinach, season with salt and pepper and cook until spinach begins to wilt.
Stir in heavy cream and Parmesan cheese and bring mixture to a simmer. Reduce heat to low and summer until the sauce is slightly reduced, about 3 minutes. Return chicken to skillet and cook until heated through, about 5 to 7 minutes.
Garnish with parsley and serve immediately with your favorite side dishes.