Preheat and prep: Heat your oven to 400°F (200°C). Lightly grease a 9x13-inch baking dish with olive oil or nonstick spray.
Season the chicken: Pat chicken breasts dry.
Sprinkle both sides with 1 teaspoon kosher salt, 1/2 teaspoon black pepper, 1 teaspoon Italian seasoning, and onion powder. Place them in the baking dish.
Make the creamy sauce: In a bowl, whisk Greek yogurt, chicken broth, milk, remaining 1 teaspoon Italian seasoning, garlic, crushed red pepper (if using), and the zest of half the lemon. Stir in parmesan until smooth.
Add veggies and tomatoes: Fold chopped sun-dried tomatoes and spinach into the sauce.
The spinach will look like a lot but wilts in the oven.
Assemble: Pour the sauce around and over the chicken, coating evenly. Drizzle with 1 tablespoon olive oil for a glossy finish.
Bake: Cover the dish loosely with foil and bake for 15 minutes. Remove foil and bake another 10–15 minutes, until the chicken reaches 165°F (74°C) in the thickest part.
Finish with lemon: Squeeze 1–2 teaspoons fresh lemon juice over the bake.
Taste the sauce and adjust salt and pepper as needed.
Rest and serve: Let rest 5 minutes so the juices redistribute. Garnish with chopped basil or parsley. Serve with whole-grain pasta, cauliflower rice, or roasted vegetables.