Using the grating attachment on your food processor, grate the potatoes and onions. Once grated, remove as much liquid as possible. I use a strong nut milk bag, but you can also just press down on the potatoes and onions in a fine mesh sieve, or pop into a clean dish cloth and twist tightly. Be sure to remove as much liquid as possible
Pop the potato mixture into bowl and add the oregano, thyme, and salt and pepper to taste
Heat 2 tbsp of cooking oil in a skillet on medium heat, until smoking slightly. Once heated, add half of the potato mixture, and use a spatula to flatten.
Add the diced mozzarella and the sundried tomatoes in an even layer, and then add the rest of the potato mixture on top. Once again, flatten with a spatula.
Cook for 2-3 minutes and then lower the heat slightly. You will still want to hear the potatoes lightly sizzling, but you don’t want them to be burning. Cook for about 12 more minutes or until the underside is nicely browned and the potatoes become tender and crispy.
Flip your rosti. To do this, place a plate over the pan and carefully flip the rosti onto the plate, then slide the rosti from the plate back into the pan, and cook for another 10 minutes (when ready the centre should be tender - you can check this with a fork).
Remove the rosti from the pan and allow it to cool slightly before serving.
While the rosti is cooking prepare your optional toppings, ready for serving (see notes).