Preheat the oven to 200C/gas 6/fan 180C. Cut the sweet potatoes into wedges and put them in a large roasting tin. Drizzle over 2 tablespoons of the oil and toss the wedges to coat. Roast for 40-45 minutes until cooked through and crisp.
Halfway through the cooking time, heat the remaining oil in a frying pan. Add the sausages and cook for 20 minutes, turning regularly until cooked through.
Drain off any fat and add the onion marmalade, with about 100ml/31⁄2fl oz water. Stir well then heat through.
Sprinkle the chips with a little salt and serve with the sausages and gravy.
Making onion marmaladeIf you can’t find ready-made onion marmalade, just slice one or two big Spanish onions thinly and cook the slices very gently in a little olive oil and melted butter over a gentle heat, stirring from time to time, for about 40 minutes. Stir in a spoonful or two of balsamic or sherry vinegar, season with black pepper and cook for 10 minutes more.