Preheat oven to 350 degrees.
To a Dutch oven or oven safe skillet add 1 tsp of olive oil and the onions. Cook the onions for about 3 minutes over medium-high heat. In the same pan, brown the ground beef. Drain the hamburger meat (as needed) and set aside.
1 pound Ground Beef, ½ Onion Diced
Melt butter in the same pan over medium heat. Add flour and combine into a paste. Cook constantly stirring for one minute.
2 Tablespoon Butter, ¼ cup Flour
Add the half and half and beef broth. Whisk together and simmer until the sauce thickens for about 7 minutes.
1 cup Half & Half, 1 cup Beef Broth
Add the salt, black pepper, onion power and garlic powder and incorporate into the sauce.
½ teaspoon Garlic Powder, ½ teaspoon Onion Powder, ½ teaspoon Black Pepper, ¾ teaspoon Salt
Add the beef mixture, green beans and sour cream to the sauce. Stir to combine. If you do not have an oven safe skillet, transfer the beef and bean mixture into a casserole dish.
2 cans Cut Green Beans, ¼ cup Sour Cream
Evenly spread the mozzarella and cheddar cheese in a single layer over the green bean mixture.
½ cup Shredded Cheddar Cheese, ½ cup Mozzarella Cheese
Sprinkle fried onion straws over the top of the casserole.
¾ cup Fried Onion Straws
Baked uncovered for 30 minutes. Let the casserole stand for 5 minutes before serving.