Preheat the oven to 375°F (190°C).
Steam the broccoli florets until tender but still crisp, about 5 minutes. Drain and set aside.
In a medium skillet, heat the olive oil over medium heat. Add the chopped onion and minced garlic and sauté until the onion is translucent, about 3 minutes.
Sprinkle the whole wheat flour over the onions and garlic, stirring constantly for 1 minute to form a roux.
Gradually whisk in the low-fat milk, continuing to stir until the mixture thickens, about 3-4 minutes.
Remove the skillet from heat and stir in the low-fat Greek yogurt, salt, black pepper, and paprika until smooth.
In a large mixing bowl, combine the shredded chicken, steamed broccoli, and the yogurt sauce mixture. Mix well.
Transfer the mixture into a 9×13 inch baking dish and spread evenly.
Sprinkle the reduced-fat shredded cheddar cheese evenly over the top.
Bake in the preheated oven for 20-25 minutes, or until the casserole is bubbly and the cheese is melted and lightly golden.
Remove from the oven and let it cool for 5 minutes before serving.