Hamburger Potato Casserole Without Canned Soup

Preheat oven to 400º, grease a 9 x 13 baking dish.
In a large skillet over medium heat, add the ground beef, cooking for about 2 minutes until it starts to brown. Then add the onions, mushrooms and thyme and cook until the beef is cooked through and the mushrooms have browned, about 8 minutes. If there is extra grease, drain it. Place in a large bowl.
In the same skillet, add butter and melt over medium heat. Add flour and cook for a minute. Slowly whisk in chicken broth adding gradually. Next slowly add milk and cook until it is slightly thickened. Remove from the heat and add sour cream, Worcestershire sauce. Salt and pepper to taste.
Add chopped potatoes to the bowl with the ground beef and pour the sauce over it, stirring to combine. Add a cup of the cheese and stir. Add to the prepared casserole dish. Cover with aluminum foil and bake for 60 minutes, until the potatoes are soft but not mushy.
Remove the foil and cover with remaining cheese. Cook uncovered an additional 3 minutes to allow the cheese to melt. Let it sit for a few minutes before serving.