Preheat oven to 350 degrees.
Add ground beef and onion to a large skillet. Cook until beef is browned, no more pink. Drain and set aside.
Melt butter in same pan over medium heat.
Add flour and combine into a paste. Let cook stirring constantly for 2 minutes.
Add half and half and beef broth. Whisk together and cook until sauce thickens. About 7 minutes.
Mix in salt, pepper, garlic powder and onion powder.
Add the cooked ground beef and green beans to the sauce and combine.
Stir in sour cream.
Transfer to oven safe dish or skillet and cook for 25 minutes covered.
Top with cheese and onion straws and return to oven uncovered for 5 minutes or until cheese is melted and onion straws have browned.