Remove the sausage casings by slicing each sausage in half lengthwise. Peel off the casing with your fingers. Put the sausages in a large pan over medium/high heat, breaking them apart into a ground mixture until golden brown. Turn the heat off and pour the mixture into a bowl.
Drain extra fat from the pan and add 2 tablespoons olive oil, sage, leeks and cayenne pepper. Cook for 3 minutes and add sausage mixture back in the pan. Cook for a little longer until all ingredients are fully cooked. Turn the heat off and keep to the side.
For the creamy tomato sauce:
In a medium size pot over medium/ high heat, add 1 tablespoon olive oil, thyme, garlic and onions. Cook for 4 minutes or until onions are translucent.
Add crushed tomatoes, lower the heat and simmer for about 20 minutes, covered loosely with a lid. Add sugar and about 1 tsp kosher salt, stir and cook for another 2 minutes.
Puree the tomatoes until smooth. Add milk and butter, and stir. Add sausage mixture and turn the heat off.