Squeeze the sausages out of their casings into a bowl. Lightly wet your hands, then shape the sausagemeat into tiny meatballs. Heat the oil in a frying pan, add the sausage balls and cook for about 5 minutes, stirring frequently, until golden all over. Pour over the tomato pasta sauce and simmer for 5 minutes.
Meanwhile, cook the pasta in a large saucepan of salted boiling water according to the pack instructions until al dente. Remove from the heat, add the spinach leaves and then drain, reserving a little of the cooking water, and return to the pan.
Mix together the ricotta, mascarpone and milk in a bowl, then stir through the pasta and season well.
Spoon the tomato and sausage sauce into a medium-sized ovenproof dish. Arrange the pasta on top and cover with the mozzarella. Place in a preheated oven, 190°C (375°F), Gas Mark 5, for 15 minutes or until golden, bubbling and cooked through.