Toss together the oil and sausages in an ovenproof tin and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 12 minutes. Give the tin good shake, add the tomatoes and cook for 7 minutes more until the sausages are browned and the tomatoes are starting to wilt. Remove from the tin and keep warm.
Add the pesto to the tin with the measurement water and stir together until warmed.
Meanwhile, heat the stock in a large saucepan with salt to taste. Add the polenta and simmer for 5 minutes, stirring frequently, until thickened like mashed potatoes. Stir through the mascarpone.
Spoon the polenta on to serving plates. Top with the sausages and tomatoes and drizzle over the pesto.