Preheat the oven to 350 degrees F.
Set six (5 3/4x3-inch) mini loaf pans on a sheet tray.
In a large pot of salted, boiling water, add the rigatoni and cook, while stirring occasionally, until very al dente, about 7-8 minutes.
Reserve 1 cup of the pasta water, draining the remainder.
In a large pot over medium heat, add 4 tablespoons of the oil.
Add the onions and the garlic to the hot oil and cook, while stirring occasionally, until deeply caramelized and softened, about 15-20 minutes.
Add 1/2 teaspoon of the salt to the onion mixture and stir to combine.
In a medium bowl, add the onion mixture.
In the same large pot, over medium-high heat, add 4 tablespoons of the remaining oil.
Add the mushrooms and cook, while stirring occasionally, until crispy and caramelized, about 10 minutes.
Add 1 teaspoon of the remaining salt to the mushrooms and stir to combine.
Add the onion mixture and the butter to the mushrooms and cook, while stirring, until the butter is melted.
Add the flour to the mushroom mixture and cook, while stirring frequently, until it is lightly golden, about 2 minutes.
Reduce the heat to medium-low.
Add the heavy cream and the Dijon mustard and cook, while whisking, until no lumps remain.
Bring the sauce mixture to a gentle simmer.
Transfer the sauce mixture from the heat.
Add the Gouda to the sauce mixture and stir until combined and melted.
If the sauce mixture seems too thick, add the reserved pasta water, a little at a time, until the desired consistency is reached. You may not need all or any of the reserved pasta water.
Add the pasta to the sauce mixture and toss until well-combined.
In a small bowl, add the breadcrumbs and the remaining 1 tablespoon of the oil and toss until the texture resembles wet sand.
Divide the pasta mixture evenly among the prepared loaf pans.
Sprinkle the pasta in the loaf pans with an even amount of the breadcrumb mixture.
Bake the loaf pans until the breadcrumb toppings are golden-brown, about 10-13 minutes.
Serve garnished with the parsley.