Preheat oven to 350 degrees F. Spray a 9x13-inch baking dish with nonstick cooking spray; set aside.
Melt butter in a large skillet over medium-high heat, add onion and sauté until tender. Mix in flour and cook for an additional 1-2 minutes. Gradually whisk in chicken broth and cook for 3 minutes, or until thickened.
Remove from heat and add milk, sour cream, yogurt, onion powder, hashbrowns and cheddar cheese; mix until well combined.
Spread mixture in prepared pan and top with bread crumbs. Spray bread crumbs generously with cooking spray.
Bake, uncovered, for 40 minutes, then turn oven to high broil and cook for 1-2 additional minutes, or until bread crumbs are golden brown.