Preparation

Make the herb dressing

• In a medium bowl, combine the basil, chives, parsley, chiles, and garlic. Add the vinegar, mustard, and capers, and then whisk in the oil. Season to taste with salt and pepper. Cover with plastic wrap directly on the surface, and refrigerate if not using within a few hours.

Roast the meat

• Trim the fat on the meat to about 1/4 inch, if necessary. Let the meat sit at room temperature for 1 hour.
• Position a rack in the center of the oven, and heat to 425°F.
• In a large bowl, toss the shallots, onions, garlic, and rosemary with 2 Tbs. of the olive oil. Season with 1 tsp. salt and 1/2 tsp. pepper.
• In a small bowl, combine the mustard and 3 Tbs. of the oil.
• Rub the meat with the remaining 1 Tbs. oil and season all over with 2-1/2 tsp. salt and 1-1/2 tsp. pepper. Place bone side down in a large heavy-duty roasting pan and roast for 15 minutes. Reduce the temperature to 350°F, brush the meat with some of the mustard mixture, and scatter the onion mixture in the pan around the beef. Roast, brushing the meat with the mustard mixture every 20 minutes or so, until a thermometer inserted into the center of the meat reads 120°F to 125°F, 2 to 2-1/4 hours more. (The roast will continue to cook with carry-over heat.)
• Reserving the onion mixture in the roasting pan, transfer the roast to a carving board. Using your hands, generously coat the meat with about half of the herb dressing; reserve the rest. Tent with foil and let rest for 25 to 30 minutes. Meanwhile, return the pan to the oven and continue roasting the onions and shallots until very tender, 15 to 20 minutes. Using a slotted spoon, transfer the mixture to a serving bowl.
• To carve, remove the meat from the bones by running a long, thin carving knife between the eye of meat and the bones along the interior curve of the bones. Slice the meat into thick (1/2 to 3/4 inch) slices. Serve with the onion mixture, and pass the reserved dressing at the table.