Coat bottom of roasting pan with olive oil and add your to onion halves and spread the rough cut carrots, celery and garlic around them.
Rinse the rack of pork and pat dry.
Place rack fat side up, on top of cut veggies and garlic, resting on the onion halves. Brush mustard over the entire top of rack of pork.
Sprinkle the herb mixed breadcrumbs over the mustard base, forming a light crust.
Roast in oven at 450°F for 15 minutes, after 15 minutes reduce heat to 325°F and continue to roast for approximately 2 hours or until internal temperature has reached internal temperature of 145°F (Medium Rare ) to 160°F (Well Done), depending on your preference.
During last 90 minutes of roasting add your other sheet pan of seasoned and oiled cut vegetables to roast.
Remove the rack of pork from the oven and place on cutting board and allow to rest for 10 minutes before slicing the meat. This will allow all the juices to remain in the rack, rather than have them run all over the plate, making the meat dry.
While the rack is resting, remove the roasted vegetable from the oven and set aside.
Feel free to deglaze the rack of pork pan with some 1 cup of chicken stock for a quick sauce, loosening the browned bits on the bottom of the roasting pan. You can strain the cooked down vegetables the rack sat on during roasting or mash into the gravy.
Cut the rack along the bones, making even portions of the pork, serve with your roasted vegetables.