Bring a large pot of salted water to a boil and add penne pasta. Cook according to package instructions, around 10–12 minutes, until al dente. Reserve ½ cup pasta water before draining.

In a large skillet, heat 2 tbsp of olive oil over medium-high heat. Season chicken breast with salt, pepper, and Italian seasoning, then cook for 5–7 minutes until golden brown and cooked through. Remove and let rest.

In the same skillet, add another tbsp of oil if needed, and sauté minced garlic and chopped onion for 3–4 minutes until the onion is translucent and fragrant.

Stir in chopped sun-dried tomatoes and red pepper flakes, cooking for 2–3 minutes to meld flavors.

Lower heat and return the diced chicken, then gently toss in cooked pasta. Adjust consistency with reserved pasta water if needed.

Add fresh spinach and stir until it wilts, taking about 1–2 minutes. Sprinkle with crumbled feta and fold in, adding lemon juice if desired before serving.