Preheat oven to 350ºF. Place the vegetables on a sheet pan and toss with vegetable oil, and salt and pepper to taste. Roast until softened but still firm and starting to brown, about 25 minutes.
Bring a large pot of salted water to the boil. Cook the ziti according to package directions to al dente. Drain and reserve.
While the vegetables and pasta are cooking, heat a large saucepan over medium low heat. Add butter, garlic and mustard seeds, if using, and cook until the garlic is aromatic, 30 seconds. Whisking constantly, stir in the flour taking care the flour doesn’t brown. Continue to cook for 2 minutes. Whisk in the milk. It will thicken as it gets hot. Whisk constantly, any lumps will smooth out. Cook for 5 – 7 minutes, mixture will bubble and thicken and when it no longer changes in consistency, it’s done. Stir in nutmeg, cayenne, Dijon and Parmigiano. Fold in the vegetables and pasta. Spread into a 9” x 13” pan. Sprinkle with the breadcrumbs. (This can be made ahead to this point and refrigerated or frozen.) Bake for 15 minutes or until heated through. Garnish with parsley.