Place potatoes on a baking tray and drizzle with oil, season and roast for 30 minutes or until golden.
In the last 10 minutes of potato cooking time, place merguez in a non-stick frypan over medium heat. Cook for 5 minutes, breaking up the meat and stirring occasionally, or until golden. Add the garlic, rosemary and half the oregano, and toss to combine, then remove from the heat.
Smash the potatoes with a fork to open out a little.
Top with a spoonful of tzatziki followed by a spoonful of the sausage mixture and scatter with the feta, mint and remaining oregano.