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1
Stir the sauce with tomato juice, horseradish, and garlic.
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2
Transfer the roast to a glass bowl or zip-lock bag.
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3
Pour the marinade over the roast and turn to coat.
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4
Marinate for at least 4 hours or up to 24 hours.
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5
Preheat the grill to high.
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6
Remove the roast to a baking sheet and reserve the marinade.
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7
Strain all but 1/2 cup of the marinade into a saucepan.
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8
Bring marinade, 1 cup of broth, and port, if using, to a boil.
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9
Reduce to heat to medium and simmer for 5 minutes.
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10
Stir the cornstarch with the remaining broth and stir into the sauce mixture.
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11
Cook, stirring until sauce is thickened; whisk in butter.
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12
Keep warm.
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13
Meanwhile, sprinkle the roast all over with salt and pepper.
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14
Transfer the roast to the preheated grill and sear each side for 1-2 minutes or until browned all over.
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15
Turn off the burner directly under the roast and reduce the temperature of the remaining burners to medium.
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16
Cook the roast, covered, for about 1 1/2 - 1 3/4 hours, turning and basting occasionally with 1/2 cup reserved marinade until beef is medium-rare or registers 140F on an instant-read thermometer.
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17
Remove the roast from the grill and rest, covered, for 10 minutes before slicing , Serve the beef with the warm gravy.
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18
For a richer flavoured sauce, stir in any juices that accumulate under the roast as it rests.
