Pre-heat the oven and a large baking tray to 215°C/420°F.
To prepare the chips, prick the potato skins with the tines of a fork and place them in the microwave at full power for 6 minutes to par-cook (alternatively boil in a pot of water until just soft) and allow to cool.
In the meantime, make the marinade by mixing together all the marinade ingredients in a bowl until well combined and pour into a zip lock bag.
Slice the rump into 3cm cubes, transfer to the zip lock bag with the marinade and turn to coat. Place in the refrigerator for 15 minutes to marinate but the longer the better.
Cut the potatoes into long, 1cm thick chips. Spread out onto the pre-heated baking tray, drizzle with olive oil and generously season with flaky salt. Bake for 20 to 25 minutes, turning them occasionally, until golden and crispy.
To make the tzatziki, combine the yogurt with the cucumber, garlic, lemon juice and season to taste with salt and pepper. Add more lemon juice if required. Refrigerate until ready to serve.
Remove the rump from the marinade and prepare the skewers by threading the beef cubes onto kebab sticks.
Start cooking the rump about 10 minutes before the chips are ready.Grill the skewers on a hot fire or griddle pan and turn to cook on all sides to your preferred doneness; it will take approximately 4 to 5 minutes for medium-rare which is recommended.
Squeeze a lemon wedge over the beef and sprinkle with origanum. Serve with warmed pita bread, Tzatziki and crispy homemade chips.