Preheat oven to 350 degrees F. Spray a 9x13-inch baking dish with nonstick cooking spray. Spread steamed broccoli in the bottom of the dish. Top with cooked chicken.
In a large skillet over medium heat, sauté onion in olive oil until tender (3-5 minutes). Add garlic and sauté for an additional minute. Season with salt, pepper and Italian seasoning. Slowly stir in chicken broth. Whisk in flour until mixture thickens.
Remove from heat and mix in Parmesan and Greek yogurt until well combined. Pour over chicken and spread in an even layer then top with bread crumbs.
Spray breadcrumbs generously with nonstick cooking spray. Bake, uncovered, for 30 minutes.