Cook the bacon in a large skillet until crisp. Transfer it to paper towels, chop or crumble it, and leave about 1 tablespoon of bacon fat in the skillet if desired.

Season the chicken with about 3/4 teaspoon kosher salt and the black pepper. If the breasts are very thick, slice them horizontally into thinner cutlets for more even cooking.

Heat the olive oil and 1 tablespoon butter in the skillet over medium-high heat. Sear the chicken for 4–5 minutes per side, until golden and the center reaches 165°F (74°C), then transfer it to a plate.

Reduce the heat to medium and add the remaining butter. Sauté the onion for 2–3 minutes, then add the garlic and cook for about 30 seconds, just until fragrant.

Pour in the chicken broth and scrape up the browned bits from the skillet. Simmer for 1–2 minutes so those savory bits become part of the sauce.

Stir in the heavy cream, Parmesan, Italian seasoning, red pepper flakes if using, and the remaining salt. Simmer gently for 2–4 minutes until slightly thickened, avoiding a hard boil.

Add the baby spinach and cook for 1–2 minutes, stirring until just wilted. Stir most of the chopped bacon into the sauce and reserve the rest for topping.

Return the chicken and any juices from the plate to the skillet. Spoon the sauce over the chicken and simmer gently for 1–2 minutes, then scatter the remaining bacon over the top before serving.