Heat oil in a large frying pan over medium-high heat. Add sausages. Cook, turning regularly until brown on all sides. Remove to a plate and slice into 2cm thick diagonal slices (about 4 slices per sausage). Reserve 1 tbsp of oil and discard excess

Add onion, garlic, celery and thyme, stirring for 4-5 minutes until softened. Add tomato sauce, Worcestershire sauce, stock and combined cornflour and water. Stir to combine, cover and bring to the boil. Reduce heat and simmer, uncovered, 10 minutes or until sauce has thickened. Stir in sausages and peas

Meanwhile, preheat oven to 220°C/200°C (fan-forced). Cook sweet potatoes in a large saucepan of boiling water for about 15 minutes, or until tender. Drain and return to saucepan over low heat. Add butter and milk. Mash until smooth. Stir in ¾ of the cheese. Season with salt and pepper

Spoon sausage mixture into 9 cup capacity ovenproof dish. Top with mashed sweet potatoes and swirl with the back of a spoon. Sprinkle over remaining cheese. Bake for 30-35 minutes, or until golden and bubbling