Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with cooking spray.
Blanch the broccoli florets in boiling water for 2 minutes, then drain and set aside.
Heat olive oil in a large skillet over medium heat. Sauté the onion until soft, about 5 minutes.
Add the minced garlic and cook for 1 minute until fragrant.
Whisk in the flour and cook for 1 minute to form a roux, stirring constantly.
Slowly pour in the chicken broth and milk while whisking continuously to prevent lumps.
Bring the sauce to a simmer and cook until it thickens, about 3 to 4 minutes.
Stir in the salt, pepper, paprika, and ¾ cup of the shredded cheddar cheese until melted.
Remove the skillet from heat and stir in the Greek yogurt until smooth.
In a large bowl, combine the shredded chicken, blanched broccoli, and the prepared sauce. Mix well.
Pour the mixture evenly into the prepared baking dish.
Sprinkle the remaining ¼ cup of cheddar cheese over the top.
Bake uncovered for 25 minutes, or until the casserole is bubbly and the top is golden brown.
Let it rest for 5 minutes before serving.
