To make rub: In medium bowl, whisk parsley, oregano, thyme, rosemary, garlic, Dijon mustard, oil, salt, black pepper and paprika until well incorporated.

To make prime rib roast: Pat prime rib roast dry on all sides with paper towels. Season generously with salt and pepper, to taste. Allow roast to come to room temperature about 30 minutes.

Rub Dijon herb rub all over prime rib roast and allow to stand 10 minutes.

Preheat oven to 250 F.

In large cast-iron pan, warm oil over medium-high heat.

Sear roast on all sides until golden brown, 2-3 minutes per side.

Place seared prime rib on wire rack-lined sheet pan and place in oven.

Cook until internal temperature is 10 F below desired cooking doneness.

Rest 15-20 minutes before slicing.

To make mushrooms: In large pan, warm olive oil over medium-high heat. Add diced onions and saute about 1 minute.

Add minced garlic and lightly saute until fragrant, about 20 seconds.

Add mushrooms and pinch of salt and ground pepper to pan. Saute 3-4 minutes, or until mushrooms are tender.

Add chicken stock and reduce to one-third in volume, 3-4 minutes.

Add butter and parsley to pan and saute until butter is melted and incorporated. Immediately remove from heat and season, to taste, with salt and pepper.

Serve mushrooms with prime rib roast.