Season the chicken breasts on both sides with 1 tsp of the salt and the pepper.
Heat the oil in a large skillet over medium-high heat. Add the chicken breasts and Sear until golden brown on both sides, about 5 minutes per side. Remove the chicken from the skillet and set aside.
In the same skillet, add the onion cook and ½ tsp salt. Cook until translucent, about 6 minutes, stirring frequently. Reduce the heat if onions start to scorch before they fully soften (you want them properly soft - no semi-raw onion crunch in the sauce, please). They pick up the color and browned flavor from the seared chicken, too. Yum. Add garlic and cook a minute more.
Pour in the chicken broth, and scrape the bottom of the skillet to deglaze it, lifting off any browned bits stuck to the bottom.
Add the heavy cream, diced tomatoes, dried Italian seasoning, and crushed red pepper flakes (if using). Stir well and bring the mixture to a simmer to let the sauce reduce for 5 minutes to thicken it up and let the flavors meld.
Stir in the freshly grated Parmesan cheese until melted in. Taste and add about 1 tsp more salt (add it in pinches and taste).
Return the chicken to the skillet, nestling it into the sauce. Let it simmer in the sauce until cooked through to 165ºF. Add spinach and cook until just wilted.
Garnish with fresh basil leaves just before serving.