Preheat oven to 350 degrees F (175 degrees C). Grease 1 1/2 quart casserole dish or a 9 x 13 baking pan.
In a large skillet, cook onion and ground beef over medium high heat until ground beef is well browned and no longer pink (about 10 minutes). Add seasoning to the ground beef, then green beans and tomato sauce.
Combine ingredients and then add them as the base layer in the casserole dish or pan. Spoon mashed potatoes in mounds onto the ground beef mixture layer, spread evenly if desired. Top with shredded cheddar cheese.
Bake in preheated oven 25-30 minutes or until the cheese is completely melted and the edges are slightly golden.
*This recipe will freeze for later, just allow to come to room temperature when ready to bake before serving.https://bakeitwithlove.com/hamburger-pie-casserole-recipe-with-creamy-garlic-roasted-mashed-potatoes/