8 to 24 hours ahead of time, place the roast into a non-reactive container and pour most of the marinade over top. Cover and set it into the refrigerator. Remember to reserve about ¼ cup of marinade for later.
When you’re ready to cook, set up the grill for indirect grilling. Place a drip tray underneath the grids where you will be placing the roast. Preheat the grill to 325°F.
Remove the roast from the fridge, discarding the marinade it was in. Place the roast over the drip tray, on the indirect side of the grill. Insert a digital thermometer, like Napoleon’s Wireless Digital Thermometer
, to ensure that you cook the roast to the perfect internal temperature.
Grill the roast for 45 minutes to 1 hour, about 30 minutes per pound, until an internal temperature of 135°F for rare (trust me!). Turn up the heat on the lit side of the grill, or light your infrared side burner.
Once hot, 500°F or better, sear the roast on all sides until grill marks are formed. Remove the roast from the grill and tent it under foil and a dishtowel until it reaches medium-rare to medium doneness (keep the thermometer in the roast to monitor the temp).
While you are roasting the beef, cook the potatoes in your favorite manner until tender. I recommend putting them on the warming rack of the grill, over the lit burners to roast. Once ready, mash the potatoes with the butter, salt, pepper, most of the cheddar, horseradish mayo, cream, and garlic. Divide the potatoes between four buttered ramekins (about 1 cup each), and top with the remaining cheese. Place the potatoes back on the still-hot grill to melt the cheese and keep warm.
While the roast is resting, use the drippings collected on the grill, from the roast to make gravy. If you find there aren’t enough drippings, supplement what is missing with butter. Melt in a pot over medium-low heat. Whisk in the flour; if there is still butter after adding 3 tbsp. of flour, add 1 tbsp. more.
Slowly add the marinade to the flour mixture, whisking as you do so. The flour mixture will clump up, but that is okay. Add the beef stock a little at a time until the whole mixture is smooth and the perfect gravy consistency for you, either thick and creamy, or thin and more “aux jus” like. There may be some juice from the meat that is resting. Add that to the gravy for extra flavor points.
Slice the beef and serve with the mashed potatoes and your favorite veggies, topped with your delicious gravy.