Preheat oven to 350° F and bring a large pot of salted water to a boil. Butter a 13x9 inch baking dish. Cook the pasta according to package directions. Drain and set aside.
In the same pot you used to cook the pasta, add the bacon and cook on medium heat until crisp. With a slotted spoon, remove the bacon from the pot leaving the bacon fat behind. Set aside for later. Discard all but about 1 tablespoon of bacon fat.
Add 2 Tbs of the butter along with the minced garlic to the remaining bacon grease. Cook for about 30 seconds or until fragrant. Add in the flour and and whisk until smooth. Simmer for 1 minute.
Slowly whisk in the heavy cream and water/chicken broth until smooth. Cook over medium heat, stirring constantly, until the sauce just starts to thicken. You will know its done when you start to see little bubbles around the edge of the pot and it coats the back of a spoon.
Turn off the heat. Add the Boursin and cheddar cheese, mustard, and half of the bacon. Stir until smooth. Salt and pepper to taste.
Add the drained pasta to the cheese sauce. Stir to coat all of the noodles in the cheese sauce. Once combined, pour into the prepared baking dish.
Mince the remaining bacon finely and combine it with the panko, parmesan cheese, parsley, and 2 Tbs melted butter. Layer it evenly on top of the pasta.
Bake at 350° F for 15-20 minutes or until bubbly and golden. Yields 4 servings as a main course or 8 servings as a side.