In a large skillet over medium heat, add 2 tablespoons of olive oil. Once hot, add the chicken pieces and season with salt and pepper. Cook until golden brown and cooked through, about 7-10 minutes.

Once the chicken is done, remove it from the skillet and set it aside. In the same skillet, add the diced onion and minced garlic. Sauté for 2-3 minutes until the onion is translucent and the garlic is fragrant.

Pour in the can of diced tomatoes (with juices) into the skillet. Stir well and bring to a slight simmer for about 5 minutes.

Stir in the fresh spinach and cook until wilted, about 2-3 minutes.

While the spinach wilts, cook the spaghetti according to package instructions in a separate pot. When al dente, reserve a little pasta water, drain, and add it to the skillet.

Add the cooked chicken back into the skillet with the spinach and tomatoes. Toss everything together, adding reserved pasta water a little at a time until you reach your desired consistency.

Serve hot, topping with grated Parmesan cheese if desired.