Preheat the oven to 450 degrees F and remove the upper rack to make space in the oven. Line a rimmed baking dish with foil or parchment paper and place the standing rib roast on it. If your roast is not trimmed, carefully cut the excess fat off the top to expose the rib bones. Look for the natural separation between the meat and the fat and cut along the separation. Clean the rib bones with a paring knife.
Stand the roast with the rib point up. Combine the mustard, bacon grease, thyme, garlic, 1 teaspoon sea salt, and 1/2 teaspoon cracked black pepper in a small bowl. Mix well, then rub over the surface of the standing rib roast. Insert an oven-safe meat thermometer into the center of the roast, pushing the tip in until it's directly in the middle.
Place the roast in the oven and roast for 20 minutes. Then lower the heat to 350 degrees F and continue roasting until the interior temperature reaches 125 degrees F. Remove from the oven and allow the roast to rest for at least 15 minutes before slicing.