Preheat the oven to 375°F (190°C).
Steam the broccoli florets in a steamer basket over boiling water for 5 minutes until tender-crisp. Drain and set aside.
Heat olive oil in a medium skillet over medium heat. Add the chopped onion and minced garlic and sauté for 3-4 minutes until softened.
Sprinkle the whole wheat flour over the onions and garlic and cook, stirring constantly, for 1 minute to form a roux.
Gradually whisk in the low-sodium chicken broth, stirring continuously to avoid lumps. Cook for 3-4 minutes until the sauce thickens.
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 cooked chicken, steamed broccoli, and the creamy sauce. Mix well.
Transfer the mixture to a 9×13-inch baking dish and spread evenly.
Sprinkle the low-fat shredded cheddar cheese evenly over the top.
Bake in the preheated oven for 20-25 minutes until the casserole is heated through and the cheese is melted and slightly golden.
Remove from oven and let it cool for 5 minutes before serving.