Cook the pasta in salted water according to the package directions. (Note 1) Reserve 1 cup of pasta water before draining. Set aside.
While the pasta cooks, heat a non-stick skillet over medium heat. Melt 1 tablespoon butter.
Season the chicken with salt and pepper on both sides. Add the chicken to the. Cook for 5-6 minutes until golden brown on the first side.
Flip the chicken, cover the pan, and cook for another 5-6 minutes until the chicken reaches an internal temperature of 165℉. (Note 2) Transfer the chicken to a plate and reduce the heat to low.
Add the pesto to the skillet along with ½ cup of the pasta water. Stir to combine and simmer for 1 minute. Add the pasta and toss to coat with the sauce, adding more pasta water if necessary. Add the baby spinach and toss to combine.
Slice the chicken and add back to the pan. Garnish with fresh tomatoes, basil, and Parmesan shavings.
Notes
Cook the pasta in well-salted water. Use 2 teaspoons salt for 12 ounces of pasta.
For thicker chicken breasts, cover the skillet after flipping the chicken. This will help to cook the chicken evenly throughout. Always use an instant-read thermometer to ensure the chicken has reached a safe internal temperature.