Slice your onion, and chop other veggies.
Add all the vegetables, except the mushrooms, to an oven proof dish with a lid.
Splash on a little soy sauce: you don’t need much. Stir through any spices you fancy, then pop the lid on and cook at 180C for about half an hour. Keep an eye on it. As soon as the veggies start to release some liquid (after about 20 minutes or so), take the lid off.
Once all the veg is pretty much cooked, add the mushrooms. Pop the dish back in the oven with the lid off. Cook for about 10 minutes more, or until the mushrooms are cooked.