Cut the tomatoes and zucchini into 1/2 cm thick slices. Peel the potatoes and cut into thin slices. Cut the onion into slices. Layer the vegetables in a flat, oiled baking dish in one layer, alternately like roof tiles. Season with salt and a lot of pepper. Sprinkle with oregano and drizzle generously with olive oil.
Bake at 180 ° C for 50-60 minutes.
Drizzle with the escaping juices several times while baking, destroying the arrangement as little as possible. If the gratin gets too dark, cover with foil.