In a medium bowl, stir together the beef stock, sherry, soy sauce, salt and pepper. Season the meat with curing salt and steak seasoning, then place it in the bowl with the marinade. Cover and refrigerate for 2 to 3 hours.
Heat one tablespoon of oil in a large frying pan over medium-high heat. Use tongs or a slotted spoon to transfer the venison to the frying pan, reserving the marinade. Cook and stir until browned and cooked through, 3 to 5 minutes, depending on the size of your pieces. Remove to a clean bowl.
Heat the remaining tablespoon of oil in the same frying pan over medium-high heat. Add the onion and pepper strips; cook and stir until starting to become tender, 2 to 3 minutes. Remove them from the pan and place them in the bowl with the meat.
Dissolve the cornflour in 2 tablespoons of the reserved marinade. Pour the rest of the marinade into the frying pan. Bring to the boil, then simmer over medium heat for a minute. Stir in the cornflour slurry and continue to cook, stirring slowly until thickened, about 1 minute. Return the meat and vegetables to the frying pan and stir until coated with the sauce. Transfer to a serving dish.