Preheat oven to 350 degrees F. Spray a 9x13 inch baking dish with non-stick spray. Set aside.
Combine chicken broth and 1/4 cup milk in a medium saucepan over medium-high heat. Bring to a low boil. Reduce heat to medium-low.
Meanwhile, in a small bowl, whisk the flour and seasonings together. Pour in remaining milk. Whisk until smooth and no lumps remain. (Mixture will be thick.)
Pour the flour/milk mixture into the saucepan with the broth mixture. Whisk and stir constantly as mixture simmers. Continue to let simmer until mixture is smooth and thick. (Around 5 minutes or so.) Remove from heat and let stand about 10 minutes to slightly cool.
In a large bowl, add cooked broccoli, soup mixture, yogurt and onion. Gently mix to combine everything. Pour mixture into prepared baking dish. Spread out evenly. Top with grated cheddar cheese.
Bake in preheated oven, uncovered, for 20 minutes or until hot, bubbly and cheese is melted. Remove from oven and serve warm. Enjoy!