Cook the pasta: Bring a large pot of salted water to a boil. Cook spaghetti to just shy of al dente (1–2 minutes less than package). Drain and set aside.

Prep the oven and pan: Heat oven to 375°F (190°C). Lightly spray a 9x13-inch baking dish with nonstick spray.

Sauté the vegetables: In a large skillet, warm olive oil over medium heat. Add onion, bell pepper, and mushrooms with a pinch of salt. Cook 6–8 minutes until softened and most moisture cooks off. Stir in garlic and cook 30 seconds.

Build the sauce base: Add crushed tomatoes, tomato sauce, and chicken broth. Stir in Italian seasoning, smoked paprika, red pepper flakes if using, 1/2 teaspoon salt, and several grinds of pepper. Simmer 5 minutes to meld.

Make it creamy the lighter way: Reduce heat to low. Whisk in cream cheese until melted and smooth. Remove from heat and cool 2–3 minutes so the yogurt won’t curdle. Stir in Greek yogurt until the sauce is silky.

Add chicken and greens: Fold in the shredded chicken and chopped spinach. If the sauce seems too thick, splash in 2–4 tablespoons broth. Taste and adjust seasoning.

Combine with pasta: Add cooked spaghetti to the skillet and toss until every strand is coated. You want the mixture slightly saucier than a typical pasta dish because it will absorb during baking.

Assemble the bake: Transfer to the prepared dish. Top with mozzarella and Parmesan for a light, even layer.

Bake: Cover loosely with foil and bake 15 minutes. Remove foil and bake 10–12 minutes more, until cheese is melted and lightly golden at the edges.

Rest and serve: Let the casserole rest 5–10 minutes so it sets slightly. Garnish with chopped parsley or basil. Slice and serve warm.