Pat chicken breasts dry and season both sides generously with salt and pepper.
Heat olive oil in a large skillet over medium-high heat until shimmering.
Sear chicken breasts for about 5 minutes per side until golden brown and cooked through; remove and set aside.
Reduce heat to medium; add diced onion and sauté until translucent and fragrant, about 3-4 minutes.
Stir in minced garlic and cook for 30 seconds, watching carefully to avoid burning.
Pour in diced tomatoes with juices, scraping browned bits from pan bottom.
Add chicken broth, Italian seasoning, salt, and pepper; simmer for 5 minutes to meld flavors.
Slowly stir in heavy cream and grated Parmesan cheese; cook on low, stirring occasionally, until sauce thickens, about 3-5 minutes.
Return chicken breasts to skillet, spoon sauce over top; cover and simmer gently for 5 minutes.
Just before serving, stir in fresh chopped basil leaves.
