First, grill the grapefruit. Add a bit of cooking spray to a grill pan, then add the grapefruit, cut side down. Cook on medium heat, and check after a few minutes to see if grill marks appear. This should only take about 3-5 minutes, depending on the heat of your stove.
Make the cinnamon honey
As the grapefruit grill, combine the cinnamon and honey in a small bowl.
Toast the pistachios
In a shallow pan over medium heat, toast the pistachios until fragrant. Toss the pan repeatedly, so they don't burn. This should only take about a minute or two.
Plate
When the grapefruit are done, plate them and drizzle with the cinnamon honey and toasted pistachios.