Cook the noodles according to the package directions, either in a skillet, where you will prepare the rest of the dish, or in a separate pot. Reserve 1/2 cup of the cooking water and set aside. Drain the pasta in a colander and rinse under cold water. Set aside.
In a skillet, heat the oil and cook the onion for a few minutes, until it starts to soften. Add the chicken, season to taste, and cook for a few minutes more, until almost cooked. Add a little of the reserved cooking water to loosen brown bits from the bottom of the skillet. Start adding spinach, in batches, and cook, stirring, until it has wilted.
Return the cooked pasta to the skillet. Add the feta and lemon juice. Toss until feta has melted a little bit. Enjoy!
1. If you prefer some heat, feel free to add red pepper flakes at the very end.