Preheat oven to 350° and set 6 (5 ¾" x 3 ¾") mini loaf pans on a sheet tray. In a large pot of boiling salted water, cook rigatoni, stirring occasionally, until very al dente and white part of pasta shows when cut, 7 to 8 minutes. Reserve 1 cup pasta water before draining.

Meanwhile, in a large pot over medium heat, heat 4 tablespoons oil. Add onions and garlic and cook, stirring occasionally, until deeply caramelized and softened, 15 to 20 minutes; season with ½ teaspoon salt. Transfer to a medium bowl.

In same pot over medium-high heat, heat 4 tablespoons oil. Add mushrooms and cook, stirring occasionally, until crispy and caramelized, about 10 minutes; season with 1 teaspoon salt. Add onion mixture and butter and stir until butter is melted. Add flour and cook, stirring frequently, until lightly golden and fragrant, about 2 minutes.

Reduce heat to medium-low. Add cream and Dijon and whisk until no lumps remain. Bring mixture to a gentle simmer, remove from heat, and add cheese, stirring to melt and combine. If mixture is too thick, add ½ to 1 cup pasta water until desired consistency is reached. Fold in pasta.

In a small bowl, toss bread crumbs and remaining 1 tablespoon oil until texture resembles wet sand.

Divide pasta among loaf pans. Top with bread crumbs.

Bake pasta until top is golden brown, 10 to 13 minutes. Top with parsley.