In a food processor, finely chop the cranberries, parsley, breadcrumbs and garlic.
Scatter 60 ml (1/4 cup) of the persillade on the bottom of a Pyrex baking dish. Microwave on medium power for about 1 minute. Set aside. Place the remaining uncooked persillade on a plate.
With the rack in the lowest position, preheat the oven to 190°C (375°F).
Salt and pepper the roast. Brush with mustard, then roll in the uncooked persillade. Place the roast on a rack in a roasting pan. Roast until a meat thermometer inserted in the centre reads 48°C (118°F) for rare, about 1 hour. Allow about 20 minutes for each 454 g (1 lb). Let the roast rest uncovered at room temperature for about 10 minutes before carving.
Finely slice the roast, cutting across the grain. Serve with Cranberried Demi-Glace Sauce (see recipe, page 60), Puréed Parsnips (see recipe, page 60) and Zucchini Ribbons (see recipe, page 56). Sprinkle the sliced meat with the cooked persillade.
Can be made a day ahead. Cook just before serving.