Preheat your oven to 400.
Peel and slice your potatoes into 1/4" circles.
Spread potatoes on a large sheet pan and toss with the oil and thyme and salt and pepper.
Roast the potatoes for 30 minutes or until they are tender and just slightly brown on the edges.
While the Potatoes are roasting, whisk the eggs ( really well) with the water and salt and pepper.
Butter (or ghee) a 9 inch pie plate.
When the potatoes are done, arrange them evenly and layer them in the pie dish. You can arrange them in concentric circles if you want.
Pour the whisked eggs over the potatoes and let it set on the counter for a few minutes to let the potatoes and eggs settle into each other.
TIP: I often stop here, cover the frittata, and put in the fridge for the morning. The potatoes soak up the egg and the salt and thyme marinate into the frittata.
Turn your oven down to 375
Bake the frittata for 40-50 minutes. The frittata will puff up and brown on top. Insert a knife to check for doneness. No runny eggy stuff :)
Allow to cool slightly before slicing.