Take the rib roast out of the fridge at least 1 to 2 hours before roasting to allow it to come up to room temperature. If there is a lot of fat on the topside of the rib roast then trim some of it off. Tie the roast two or three times with twine to hold it together. Pat the roast dry with paper towels.
Finely mince 3 garlic cloves. Finely chop fresh rosemary, thyme, and sage. In a small bowl, combine the garlic, herbs, kosher salt and pepper. Add 3 tbsp olive oil and stir together. Set aside.
Preheat the oven to 300° F. Scatter the sliced onion and sprigs of fresh rosemary, thyme, and sage in the bottom of a roasting pan. Drizzle with olive oil and cover with a rack. Place the roast on the rack with the rib side up. Rub the bottom with some of the herb rub then turn the roast over. Cover the top and sides of the roast with the remaining herb rub.
Roast the beef for 2 to 2 1/2 hours until it registers an internal temperature of 120° F in the very centre of the roast. Remove from the oven and tent the roast with foil. Let rest for 30 minutes.
In a small saucepan, heat 1 tbsp olive oil over medium heat. Add the shallot and sauté for 2 to 3 minutes until softened. Slice the remaining garlic clove and add to the pan. Cook for another minute and then deglaze with the red wine. Add a sprig of fresh thyme and the bay leaf.
Bring the wine to a simmer and let reduce by half. Add the beef broth and continue simmering until the broth reduces to around 3 cups. Strain the broth and set aside to use for the gravy.
Scoop the marrow out of two of the marrow bones and cut into small cubes. Store in the fridge until ready to use.
Turn the heat up to 425° F. Arrange two of the marrow bones around the rested prime rib. Place the roast back in the oven and roast for another 5 to 10 minutes until the beef develops a browned crust, or longer if you prefer it more well done. Remove the roast to a platter and tent with foil. Continue roasting the bones until the marrow is soft and lightly bubbling, another 5 to 10 minutes.
Remove the rack and sprigs of herbs from the roasting pan. Drain all but 3 tablespoons of fat from the roasting pan. Place the pan over medium high heat. Use a whisk to scrape up any brown bits.
Add the flour to the roasting pan and whisk into the fat to make a roux. Let the roux cook for 2 to 3 minutes. Add the reduced beef broth, half a cup at a time, whisking constantly to prevent lumps. Bring the gravy to a boil and cook until thickened.
Scoop out the roasted marrow from the bones and add to the gravy. Whisk the marrow into the gravy and then check the seasoning. Strain the gravy through a fine mesh strainer into a sauce pan. Bring the gravy to a simmer and then add the diced fresh marrow. Poach the marrow gently for a minute or two just until cooked through.
To carve the prime rib roast, begin by removing the strings. Place the roast with the bones side down on a cutting board. Using a sharp carving knife, cut along the bones to separate them from the roast and set aside. Slice the roast into slices of your desired thickness. Serve the prime rib with the bone marrow gravy and the sides of your choosing. Enjoy!