Preheat oven to 400˚F.
Place a large pot of water over high heat and bring it up to a boil to cook the pasta. Add salt and the pasta, and cook just shy of al dente.
While the pasta is cooking, heat a medium-size skillet over medium heat and add bacon. Cook bacon until crispy, about 5 minutes, then add leeks and cook until tender, about 2 minutes more. Add flour and stir, cook 1 minute. Whisk in stock, milk, salt and pepper, and bring up to a bubble. Stir in the cheese in a figure-eight motion, reserving 1/2 cup for garnish.
Drain past and combine with the sauce. Spoon pasta into tomato shells and garnish with a sprinkle of the remaining cheese and breadcrumbs. Place tomato shells on a baking sheet and bake for 12-15 minutes until golden brown on top.