Boil the tortellini. Bring a large pot of salted water to a boil. Cook the tortellini according to package directions until just tender. Reserve 3/4 cup of the pasta water, then drain and set aside.
Season the chicken. Pat the chicken dry.
Toss with 1/2 teaspoon salt, pepper, garlic powder, and onion powder.
Sear the chicken. Heat olive oil in a large skillet over medium-high. Add chicken in a single layer. Cook 5–7 minutes, stirring once or twice, until cooked through and lightly browned.
Transfer to a plate.
Build the sauce base. Reduce heat to medium. Add pesto to the skillet and stir for 30 seconds to warm it. Pour in 1/2 cup reserved pasta water and whisk to loosen into a sauce.
Make it creamy. Lower heat to medium-low.
Whisk in Greek yogurt until smooth. If it looks too thick, add a splash more pasta water. You want a glossy, pourable sauce.
Add the good stuff. Stir in Parmesan and spinach (if using).
Let the spinach wilt, about 1 minute. Squeeze in lemon juice if you like a brighter flavor.
Combine and finish. Add the cooked tortellini and chicken to the skillet. Toss gently until coated and heated through.
Taste and adjust with a pinch of salt, extra Parmesan, or red pepper flakes.
Serve hot. Plate it up and finish with a little more Parmesan and cracked black pepper.