Preheat oven to 350°F (175°C).
In a large bowl, combine ground beef, bread crumbs, egg, 1/4 cup milk, onion, garlic powder, salt, and pepper.
Form into 6–8 hamburger patties.
In a skillet, heat oil or butter over medium heat and brown patties on both sides (they don’t need to be fully cooked through).
Arrange patties in a greased 9x13-inch baking dish.
In a separate bowl, whisk together cream of mushroom soup, 1/2 cup milk, sour cream, and thyme.
Pour sauce over the patties in the baking dish.
Bake uncovered for 30–35 minutes until hot and bubbly.
Let rest 5 minutes before serving.
Garnish with chopped parsley if desired.