In a medium-large mixing bowl, add all of the ingredients for the steaks, EXCEPT the butter and olive oil. Try not to over-mix.
Divide the meat into 6 equal-sized pieces and form into circles or oval patties.
In a large skillet over medium-high heat, add the butter and olive oil.
Add the steak patties and fry both sides until a brown crust develops, remove from the pan and set aside (leave the pan drippings in the pan)
To the pan, add the onions and mushrooms and saute until the onions are golden.
Add the steak patties back to the pan and pour in the wine and the beef broth, bring to a boil and then lower to a simmer.
Cover with a lid and simmer for 30 minutes (this is the perfect time to get your side dishes going)
After 30 minutes, remove the patties.
In a small bowl, mix the cornstarch with 3 tsp of water. Drizzle this mixture into the broth while stirring the broth.
Turn the heat up to medium-high and continue stirring until the broth turns into a thick gravy.
To serve, spoon the gravy, along with the mushrooms and onions, over the Salisbury steaks.