Halve chicken breasts the long way, so that you have two thin pieces of chicken breast (see video for a visual) -- this helps them to cook more evenly. (You can also use 4 chicken breasts flattened to the same thickness, but you will need a large skillet to cook them)
Heat oil in a large skillet over medium-high heat. Season chicken breasts with salt and pepper.
Brown chicken breasts on each side. This will take about 2-4 minutes per side.
Remove chicken breasts from the pan and keep covered.
Add the garlic, salt, basil, pepper and pepper flakes if using. Cook 1 minute -- do not let the garlic burn!
Add diced tomatoes and tomato sauce. Scrape and browned bits off the bottom of the pan with a wooden spoon.
Whisk together the cream and corn starch and add to the pan. Stir to combine.
Add the chicken breasts back to the pan. Bring to a low simmer, then reduce the heat to medium-low and cover. Cook until chicken breasts reach an internal temperature of 165 degrees F and the sauce has thickened slightly. Serve.
