Cut chicken breasts in half lengthwise or pound until they are ½ inch thick. Season on both sides with salt and pepper.

Add ¼ cup flour to a shallow bowl. Dredge chicken in flour and transfer to a sheet tray.

Heat 1 tablespoon oil in large cast iron skillet over medium high heat. Add chicken and cook until golden brown and fully cooked through, 5 minutes per side. Remove to a plate and set aside. Wipe out skillet.

In same skillet, reduce heat to medium and add remaining 1 tablespoon oil. Add onions and cook until soft and translucent, 5 minutes. Add garlic and cook until fragrant, 1 minute. Add remaining 2 tablespoons of flour and incorporate with a wooden spoon. Start with just a small drizzle, slowly incorporate chicken broth, mixing between additions.

Once all the broth is added, mix in pesto and milk. Add tomatoes. Bring sauce to a simmer and cook until thickened, 5 minutes. Reduce heat to low and add chicken back in. Cook until warmed through, 2 minutes. Garnish with fresh basil and serve immediately.