Heat oil in a large skillet over medium-high heat. Add onions and cook, stirring often until softened, about 2 minutes. Add ground beef and cook, crumbling with a spoon until browned and crumbled, about 5 minutes.
Add ranch seasoning, au jus mix, pepperoncini, pepperoncini brine, water, and pepper, and stir to combine. Bring mixture to a simmer. Reduce heat and simmer until thickened and saucy, 5 to 7 minutes.