Cook the pasta: Bring a large pot of salted water to a boil. Cook the high-protein pasta until just shy of al dente (about 1–2 minutes less than package directions).
Reserve 1 cup of pasta water, then drain.
Season the chicken: Pat chicken dry. Season with 1 teaspoon salt, 1/2 teaspoon black pepper, Italian seasoning, and onion powder. Toss to coat.
Sear the chicken: Heat olive oil in a large skillet over medium-high.
Add chicken in a single layer. Cook 5–7 minutes, flipping once, until golden and cooked through. Transfer to a plate; tent with foil.
Build the garlic base: Reduce heat to medium.
Add butter to the same skillet. Once melted, add minced garlic and red pepper flakes (if using). Stir 30–60 seconds until fragrant; avoid browning.
Deglaze and simmer: Pour in chicken broth and scrape up browned bits.
Let it bubble for 2–3 minutes to reduce slightly.
Make it creamy: Lower heat to medium-low. Whisk in milk. Stir in Parmesan until melted and smooth.
Finish the sauce: Remove the skillet from heat briefly.
Whisk in Greek yogurt until fully combined. Return to low heat and warm gently. Do not boil to prevent curdling.
Season and brighten: Add lemon juice (and zest if you like).
Taste and adjust salt and pepper. If the sauce is thick, add reserved pasta water a little at a time until silky.
Combine: Add chicken back to the skillet with any juices, then add the drained pasta. Toss over low heat until the sauce coats every piece, 1–2 minutes.
Add a splash more pasta water if needed.
Serve: Top with chopped parsley and extra Parmesan. Serve hot.