Step 1 Pre-heat oven to 200’C/180’C fan/gas mark 6. Step 2 Bake sausages on a baking tray for 20 minutes turning halfway through cooking. Remove and when cool enough slice into 1’ thick slices. Step 3 Cook potatoes in boiling water for 7 minutes. Drain. Step 4 Melt butter in a large frying pan, add onion and leeks, sage and thyme. Cook until softened, about 5 minutes. Step 5 Add flour, cook for 1 minute, then add milk, whisking continuously, until sauce thickens, boils and is smooth. Step 6 Remove from heat, add 75g (3oz) cheese and stir until melted. Step 7 Transfer the sliced sausage to an ovenproof dish, top with the cheese, onion and leek sauce. Step 8 Scatter the potato slices on top, sprinkle over remaining cheese. Bake for 30 minutes until browned.