Cook the pasta. Bring a large pot of salted water to a boil. Cook the pasta until just shy of al dente. Reserve 1 cup of pasta water, then drain.

Season the chicken. Pat the chicken dry. Toss with 1 teaspoon salt, 1/2 teaspoon black pepper, Italian seasoning, and onion powder.

Sear the chicken. Heat 1 tablespoon olive oil in a large skillet over medium-high. Add half the chicken in a single layer. Cook 3–4 minutes per side until golden and cooked through. Remove to a plate and repeat with remaining oil and chicken. Do not overcrowd the pan.

Sauté the garlic. Reduce heat to medium. Add butter to the empty skillet. Once melted, add garlic and red pepper flakes. Cook 30–60 seconds until fragrant, stirring frequently. Do not brown the garlic.

Build the base. Whisk in chicken broth, scraping up browned bits. Simmer 2 minutes.

Thicken the sauce. In a small bowl, whisk milk with cornstarch until smooth. Pour into the skillet, whisking constantly. Simmer 2–3 minutes until slightly thickened.

Add yogurt and cheese off heat. Turn heat to low. Whisk in Greek yogurt until smooth, then gradually add Parmesan. Stir until melted and silky. If it seems too thick, loosen with reserved pasta water, a splash at a time.

Brighten and season. Stir in lemon juice. Taste and adjust salt and pepper. You want a balanced, savory, slightly tangy sauce.

Add greens and chicken. Fold in spinach and the cooked chicken (and any juices). Let spinach wilt for 1–2 minutes over low heat.

Combine with pasta. Add drained pasta and toss well. Use additional pasta water to create a glossy, clinging sauce. The sauce should coat the noodles without pooling.

Finish and serve. Top with extra Parmesan and chopped parsley. Serve hot.