Preheat the oven to 200°C/gas mark 6.
In a large bowl, toss together all of the ingredients. Spread the mixture in a single layer on a baking sheet. Bake until the potatoes and onions are golden brown and cooked through, about 40 minutes. Serve immediately or cool and store in an air-tight container in the refrigerator, reheating the next day in a preheated 200°C/gas mark 6 oven for 10 to 15 minutes.