Sear sirloin roast about 2-3 minutes per side. Be aware, when you're searing the fat cap side, your grill may flame up from the dripping fat.
Reduce heat to low and cook roast over indirect heat. Make sure your sirloin's fat cap side up for this step so as the fat renders, it melts into the meat for more flavor.
Allow sirloin roast to cook until center reaches 132-140 degrees for medium rare. You can test this with a digital meat thermometer.
Remove from grill and tent with aluminum foil. Let meat rest about 10 minutes so the juices can redistribute and meat will finish cooking somewhat (if you slice too early, the juices will run out and the meat will be dry).
Slice meat across the grain into medallions and serve with Wasabi Mayo.
Notes
To enhance the flavor, use a smoker box with applewood chips. This is definitely an optional step but it makes a big difference.
Recipe by Pastured Kitchen at https://www.pasturedkitchen.com/recipes/grilled-smokey-coulotte/