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1
Using electric mixer, beat creme fraiche and cream in medium bowl to firm peaks. Fold in horseradish and 1/3 cup mustard. Season to taste with salt and pepper. Cover and chill. DO AHEAD:
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2
Preheat oven to 450u00b0f. Toss onions, shallots, vinegar, oil, and 1 1/2 teaspoons coarse salt in large bowl. Sprinkle roast lightly with coarse salt and generously with pepper. Stir 2/3 cup mustard and garlic in small bowl; rub over roast. Place roast, bone side down, in roasting pan. Sprinkle mustard seeds over, pressing to adhere. Roast 20 minutes. Reduce oven temperature to 350u00b0F.
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3
Arrange onion mixture around roast. Return to oven; roast until instant-read thermometer inserted into center registers 120u00b0F, stirring onion mixture occasionally, about 2 1/4 hours for medium-rare.
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4
Transfer roast to work surface. Tent with foil; let rest 30 minutes. If necessary, increase oven temperature to 450u00b0F and return onion mixture to roasting pan and roast until deep brown and very tender, about 10 minutes. Using slotted spoon, transfer onion mixture to medium bowl.
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5
Place roasting pan over 2 burners on medium-high heat. Add brandy; stir 30 seconds. Whisk in broth and remaining 1 teaspoon mustard; boil until reduced to 1 1/4 cups jus, stirring occasionally, about 10 minutes. Season jus with salt and pepper.
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6
Slice roast; arrange on platter. Spoon onions around roast. Sprinkle parsley over. Pass dijon creme fraiche and jus alongside.
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7
The king of roasts meets its match in the king of wines. Dijon mustard's pleasant sharpness softens Cabernet's big tannins. With the roast, try the ROBERT MONDAVI 2006 CABERNET SAUVIGNON OAKVILLE (California, $45). The wine's spice and blackberry flavors are perfect with the beef.
