Add oil to a large (12-inch) skillet over medium-high heat. Once shimmering, add onion and sauté for 3 minutes, then add garlic and sauté another 30 seconds. Now add venison and cook, stirring often, for another 5 minutes.
Stir in the butter until melted.
Then stir in the ranch seasoning, au jus seasoning, ketchup, broth, pepperoncini peppers, and pepperoncini juice. Let it cook for about another 6 to 8 minutes, stirring often, until the sauce thickens.