There are so many different ingredients hamburgers can be "stuffed" with. This is a particular favorite of mine, as I love mushrooms, and the added sour cream gives it a little kick and makes them that much juicier.
The original recipe suggested ground chuck or round, but I usually use just regular ol' 70/30 ground beef.
I hope you try them and like them!
Saute mushrooms and onions together in butter in a large skillet over moderate heat until tender, about 6 to 8 minutes. Remove from skillet and mix in bowl with sour cream, basil and 1/2 teaspoon of the salt. Set aside.
The easiest way to make your patties of even and equal thickness that I have found is the following: on a large piece of wax paper, flatten the ground beef into a rectangle of approximately 12-inches by 6-inches of even thickness. Dipping a knife in cold water for easier cutting, cut the strip evenly in half length-wise, making two 12-inch lengths. Cut each length into 4 equal squares, for a total of 8 3-inch squares. *It takes longer to write out than it will to actually do, this is just my rather clumsy way of describing it!
Divide the mushroom mixture evenly onto 4 of the meat squares, to within 1/4-inch or so of the edges. Top with the remaining squares. Lightly moisten your hands with water and crimp edges together so filling will not seep out during cooking. Shape with your hands into hamburger rounds. Sprinkle both sides of patties with the remaining 1/2 teaspoon salt and the pepper.
Heat the oil in a large skillet. Pan-fry the burgers over moderate heat for about 5 minutes on each side, or until done as you like them. You can also broil or grill them, if you prefer.
When done, place between hamburger buns with your desired condiments, cheese, etc.