Preheat the oven to 200C (180C fan). Put the potato chunks in a large roasting tray and toss with the oil, salt and pepper.
Roast the potatoes for 15 minutes on their own so they start to crisp at the edges.
Add the sausages and onion wedges to the tray, tucking them between the potatoes. Roast for another 15 minutes, turning the sausages once.
Pour the chopped tomatoes and water over everything, sprinkle with the dried thyme, and return to the oven for 15 to 20 minutes until the sausages are cooked through and the tomato sauce is bubbling and thickened.
Check a sausage is piping hot all the way through with no pink remaining, then serve straight from the tray.