Start by boiling water on a pot, for the spaghetti and when it has started boiling pour them inside and add a bit of salt and mix well.
Chop the onion in small pieces and fry it on a pan with the olive oil for 2-3 minutes, then add the tomato paste, the warm water and salt and pepper, when the sauce starts boiling slice the montbeliard sausage in thick cylinders and pour it inside.
When the spaghetti is boiled pour it on a colander and then put them on a large bowl. Wait until the sauce gets a bit thick and when it is done, pour it on the spaghetti and mix well. Serve the food when it is warm.