Heat oven to 200C/180C fan/gas 6. Put the sausages in a large roasting tin with the onion, squash and sage. Drizzle over the olive oil and toss together. roast for 40-50 mins, turning occasionally, until the sausages are golden and the squash is tender. Transfer to serving plates.
Drain away any excess oil from the tin, then place the tin over a medium heat on the hob. Mix in the flour, then gradually pour in the stock, scraping up any bits left in the bottom of the tin. Stir in the mustard. Gently simmer for 5 mins until thickened. Serve with a spoonful of yesterday’s leftover mash and the pan juices.