Heat the olive oil in a large skillet over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.
Add the minced garlic and cook for an additional minute until fragrant.
Season the chicken breasts with salt, pepper, and Italian seasoning. Place them in the skillet and sear on each side for about 4-5 minutes, until golden brown.
Pour in the diced tomatoes (with their juice) and the heavy cream, stirring to combine. Bring to a gentle simmer.
Reduce the heat to low and cover the skillet. Cook for 15-20 minutes, or until the chicken is cooked through (internal temperature should reach 165°F).
Once the chicken is cooked, stir in the fresh spinach and grated Parmesan cheese. Cook for another 2-3 minutes until the spinach has wilted and the cheese has melted into the sauce.
Taste and adjust seasoning with additional salt and pepper if needed.
Serve immediately, spooning the creamy sauce over the chicken, and garnish with fresh basil if desired.