Using a mandoline, start slicing the potatoes and onion. If you don’t have a mandoline you can just slice them the usual way, by using a knife but it’s more complicated. A mandoline will save you a lot of time + all slices will have the same thickness.
Spray the casserole with some olive oil.
Place the white potato slices in the casserole in a circular way, one on top of half of the other, just like you do when you make apple pie for example. Add sliced sweet potato and onion, randomly, between the white potato slices.
In a separate bowl, whisk 2 eggs. Add grated feta cheese, salt, pepper, thyme. Pour this mixture ver the potatoes in the casserole. Using a fork, try to disperse the mixture so that most of the slices will be covered.
Place in preheated oven at about 180ºC/356ºF and cook for 30-40 minutes. Check if ready using a fork. If the potatoes are tender, it’s ready!
When ready, leave it a bit to cool. Remove from form and sprinkle with smoked paprika and fresh herbs.