For the spinach gratin, boil the potatoes, peel and cut into ½ cm thick slices. Grease a baking dish with olive oil, place a layer of potatoes like a roof tile. Salt, pepper and sprinkle with oregano.
Empty the tomatoes and their juice into a bowl and cut them into smaller pieces. Stir in some tomato paste (if you have some at home). Pour the mixture over the potatoes.
Grate half of the feta over it with a coarse grater. Finely chop the garlic and sprinkle on top as well.
Roughly chop spinach leaves a few times and also spread on the casserole. Then whisk cream and egg. Pour over and sprinkle with the remaining grated feta cheese.
Bake the spinach gratin at 180°C for about 30-40 minutes until brown.