Heat olive oil in a large skillet over medium-high heat.
Season the chicken pieces with salt and black pepper.
Add the chicken to the skillet and cook for 5-6 minutes, stirring occasionally, until golden brown and cooked through. Remove chicken from the skillet and set aside.
In the same skillet, add minced garlic and sauté for 30 seconds until fragrant.
Add the penne pasta, chicken broth, and water to the skillet. Stir to combine.
Bring the mixture to a boil, then reduce heat to medium-low. Cover and simmer for 10-12 minutes, stirring occasionally, until the pasta is tender and most of the liquid is absorbed.
Stir in the fresh baby spinach, lemon zest, lemon juice, grated Parmesan cheese, crushed red pepper flakes, and butter.
Return the cooked chicken to the skillet and toss everything together until the spinach wilts and the sauce is creamy and well combined.
Adjust seasoning with additional salt and pepper if needed.
Serve immediately, garnished with extra Parmesan if desired.