In a small bowl fold together the feta, sour cream, garlic, and dill until combined. Serve on the side with the medallions.
Slice the venison tenderloin into 1/3-inch slices.
In a shallow dish, mix together the flour, mustard powder, cayenne, salt and pepper. In a separate shallow dish, mix together the buttermilk and whole-grain mustard. Dredge the medallions in the buttermilk mixture for a minute. Then coat with the flour mixture.
In a cast iron skillet over medium-high heat, heat the oil. Fry the venison until golden brown on both sides. Place on a paper towel lined plate to drain. Serve with feta sauce.