Preheat oven to 400*F.
Slice everything thinly. Coat the onions liberally with olive oil and seasoning and put on the bottom of a 9x13 baking dish (our largest one.)
Coat the potatoes and zuccini with olive oil and seasoning, then mix in the tomatoes. Layer on top of the onions.
Bake uncovered for 1 1/2 hours. Stir halfway through.
Briam is traditionally a main dish, with the olive oil providing the bulk of the calories. It was eaten with bread and a bit of feta. It's very rich–the vegetables cook down but the olive oil remains constant in volume.
It's astonishing how much flavor this dish gets from how few ingredients. It's also astonishing how much gets devoured and how little left over, given how huge a pan you start with! But everything except the potatoes cooks down by quite a lot as moisture is driven off.
Some of the vegetables will be leathery or have smoky bits–that's intentional and it's part of the flavor.
Briam is also made with layers of many vegetables, often including eggplant. If you do this, you still "stir" by carefully poking a spoon through the layers and wiggling things a bit to loosen, while preserving the layers as much as possible. I've never tried this since I don't have a tall enough oven dish.