2 1/2tbspparsleypicked leaves with soft stalks attached,
Heat the oven to 200C (180C fan)/390F/gas 6. Put the onions, garlic, potatoes, five tablespoons of oil, 100ml water, a teaspoon and a half of salt and a good grind of pepper in a large roasting tin (roughly 40cm x 30cm).
Toss together, then bake for 35 minutes, stirring once halfway, until the vegetables have softened and started to take on some colour and the water has evaporated.
Lay the plums cut side up in the tray, add the rosemary leaves, then nestle in the sausages. In a medium bowl, whisk the vinegar, molasses, sugar, two tablespoons of water, a tablespoon and a half of sumac, two tablespoons of oil, half a teaspoon of salt and a good grind of pepper.
Pour this all over the contents of the tray, then return to the oven for 40 minutes, turning the sausages once halfway, so they cook and colour evenly.
Turn up the oven to 220C (200C fan)/425F/gas 7 and roast for 10 minutes more, until everything is nicely browned, the plums have broken down and the sauce is bubbling and sticky.
Toss the parsley with the remaining teaspoon and a half of sumac and remaining tablespoon of oil, dot all over the sausage mixture, then serve warm straight from the tray.