Heat a little oil in a large pan over medium-high heat. Sear the roast on all sides until browned, about 12-15 minutes total. Season with salt and pepper, spread mustard over the meat, top with chopped garlic, and spread plum jam over the top.
Transfer to a roasting pan and place in a preheated oven at 180°C (356°F). Roast for 30 minutes.
Remove from oven and spread onion rings over the top. Pour 2 glasses wine into the pan around the roast. Return to oven and roast for another 40 minutes, basting with the pan juices every 10 minutes, until the internal temperature reaches 63°C (145°F).
Remove the roast from the oven and let rest for 5 minutes.
Transfer the roast to a cutting board. Pour the pan juices and drippings into a saucepan. Add 1 cup whipped cream and 1 cup crème fraîche, bring to a boil over medium-high heat, stirring occasionally, about 3-5 minutes. Season to taste with herbs.