Grate the potatoes and sweet potato, add to a large mixing bowl. Using paper towel squeeze the grated vegetables to absorb the excess water. It is best to dry them off as much as possible.
Add the all-purpose flour and sea salt, toss through using your hands to lightly coat.
Place a heavy base medium fry pan on medium to high heat. Fill up to 1 1/2cm deep with vegetable oil and allow it to come up to frying temperature. To test if the oil is hot enough, add a pinch of potato mixture. If it sizzles and bubbles around the edge of the potato, then it's ready to start cooking.
Using your hands press tightly together roughly 2 tbsp of the vegetable mixture. Place it into the oil, using a spatula press down with pressure to flatten and compact the vegetables. Lightly fry and gently swirling the pan, once the colour has turned golden (a couple of minutes) and the edges are crispy gently flip over to cook the other side for a further minute or two. Once cooked on both sides, remove and place on a piece of paper towel. Cover with a lid to keep warm while the remainder are cooked.
To cook the mushrooms; placed a heavy base fry pan on medium heat, add a drizzle of olive oil followed by the mushrooms and thyme. Sauté while stirring until the mushrooms become golden and soften. Remove from the heat.
To serve; add a spoon of Persian fetta on top of each rosti, followed by add the mushrooms and a sprinkle of chives.
Servings 0