Boil a large, salted pot of water for the pasta and cook it al dente according to package directions.

Prep your chicken. Add it to a skillet, along with the olive oil, dried oregano, and garlic powder. Cook over medium-high heat for 5-7 minutes, stirring often. Chicken should be close to cooked. Take chicken out of pan and set aside.

Add the chicken broth, Dijon mustard, and lemon juice to the pan. Stir until the mustard has dissolved. Let it simmer for about a minute.

Add the tomatoes and cook for 4-5 minutes, stirring occasionally. They should be falling apart and making a nice tomato sauce. 

Add the chicken back into the pan. Let it heat through (and make sure it's fully cooked without overcooking it). 

Stir in the spinach and feta. Let the spinach wilt, and then toss the sauce with the pasta (add a little pasta water if you wish) and serve immediately. Optional: add red pepper flakes for a bit of heat and/or freshly grated parmesan.