For the marinade, pass the onion through a meat grinder or grate on a coarse grater. Or just finely chop.
Add olive oil, strongly carbonated mineral water, adjika, oregano, salt and pepper to the onion and mix.
Cut the turkey fillet into portions, lightly chop it off, pour the marinade and leave it in the refrigerator for a day.
An hour before cooking, add the ketchup and stir.
Fry the turkey fillet on the grill on both sides, for 4 minutes on each side (or bake in the oven, or fry in a frying pan in olive oil, or in a grill pan, or on a baking rack).