Place a pot of water on to boil for the pasta. When the water reaches a boil, add some salt and the pasta, and cook until al dente.
While the pasta cooks, heat a large skillet over medium heat. Add extra-virgin olive oil and chopped bacon and cook until crisp.
While bacon is cooking, split the trimmed leeks in half lengthwise, lay the leeks cut side down and thinly slice into half-moons. Fill a large bowl with water and mix the leeks into the water.
Allow the water to settle and the dirt and the grit to settle to the bottle of the bowl. Using your hands, draw the leeks from the water, taking care not to unsettle the dirt.
Drain the cleaned leeks on a kitchen towel, pat dry and, and then add to the pan with the bacon. Season the leeks with salt and pepper. Cook until leeks are tender, about 3 to 4 minutes.
To start the cheese sauce, heat a medium sauce pot over medium heat. Add the butter and melt, then add the flour, cayenne, and paprika and whisk together over the heat until the roux bubbles; cook for 1 minute more.
Whisk in the milk and stock and raise the heat a little. Bring the sauce to a quick boil and simmer to thicken, about 5 minutes.
Remove the sauce from the heat and whisk in 3 cups of the grated cheese and the mustard. Set aside.
Add the cherry tomatoes to the leeks and continue to cook for 2 minutes. Remove from the heat and reserve.
Preheat the broiler.
Drain the pasta and add it back to the pasta pot. Combine the reserved leek mixture and the cheese sauce with the pasta. Season with salt and pepper to taste.
Transfer the pasta to a baking dish. Combine the remaining ½ cup of grated cheese with the bread crumbs.
Top the pasta with the mixture. Place the dish under the broiler until the cheese melts and the bread crumbs are brown.