Add the olive oil to a large saute pan over medium heat. Add the onions and sausage and cook stirring frequently until lightly browned. Add the garlic and cook for about 30 seconds.
Add the chicken stock, chopped tomatoes (undrained), milk, pasta, and seasonings. Bring the mixture to the boil, cover, and reduce heat to low. Simmer for about 15 minutes, or until pasta is tender and most of the liquid is absorbed.
Turn off the heat and stir in 1/2 of the cheese. Sprinkle the remaining cheese on top and cover for about five minutes to allow the cheese to melt.
Top with the sliced spring onions and pop under the grill for a couple of minutes to melt the cheese.