1. 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. Heat oil in a large skillet over medium-high heat. Season chicken breasts with salt and pepper.
2. Brown chicken breasts on each side. This will take about 2-4 minutes per side. Remove chicken breasts from the pan and keep covered.
3. Add the garlic, salt, basil, pepper, and pepper flakes if using. Cook 1 minute — try not to let the garlic burn. Add diced tomatoes and tomato sauce. Scrape and browned bits off the bottom of the pan with a wooden spoon.
4. 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 °F (74 °C) and the sauce has thickened slightly.