Boil salted water and cook pasta until al dente. Reserve 1/2 cup of pasta water before draining.
Season the cubed chicken breast with salt, pepper, and oregano.
Heat olive oil in a large skillet. Add chicken and sear until golden brown. This takes 7-8 minutes.
Remove chicken from the pan and set aside.
Reduce heat to medium. Add garlic and red pepper flakes. Sauté for 60 seconds.
In a bowl, whisk yogurt, broth, lemon juice, and zest.
Lower heat to low and add yogurt mixture. Whisk for 2-3 minutes. Do not let it boil.
Stir in the parmesan cheese until it melts.
Add the baby spinach and stir until it wilts.
Add pasta and chicken to the pan. Toss to coat well. Use reserved water if needed.
Garnish with fresh parsley and lemon zest before serving.