In a large bowl, mix together all ingredients (except for butter and oil) and form into 8 patties.
Heat oil and butter in a pan at medium heat and cook patties on both sides until browned on outside, but no longer pink in middle (roughly 5-8 minutes each side).
Add onions to skillet where meat was cooked and sauté until soft at medium-high heat (about 5-10 minutes).
Add next four ingredients and whisk together.
Mix cornstarch with reserved beef broth and add to gravy to thicken.
Add patties back to pan until heated through.
Spoon gravy over Salisbury steaks.