Fire up the grill, prepare it for indirect heat. Have it running at 150-180° C / 300-350° F. This is what I call 'medium' heat.
When the grill is up to temp, simply place the butternut on the glowing embers and close the lid.
Rotate the pumpkin by one third when 15 minutes have passed. Do this again after another 15 minutes, and when it's been on for a total of 45 minutes (two rotations), it's done. The reason to cook it on three 'sides' instead of just two is to cook it more evenly.
Bring the pumpkin inside and slice it lengthwise. Now you can either slice across in thick slices (1-1.5"), like you slice bread, or cut it into large cubes.
Sprinkle an even layer of sumac on the pumpkin meat, followed by crumbled feta cheese and roughly chopped pistachios, and finally drizzle the maple syrup over it.