Sprinkle the shrimp with salt and pepper. Place a grill pan over medium heat and heat the oil. When the oil is hot, par cook the shrimp by placing them in a single layer across the grill slats and cook for 1 1/2 minutes. Flip and cook for 30 to 45 seconds. The shrimp will start to turn pinkish white. Take them out of the pan before the tails curls. Place them aside and cook the rest. Add more coconut oil as needed.
In the same pan, add a little more coconut oil and grill the whites of the spring onions until golden brown on both sides, 1 to 2 minutes total. Remove from the pan and allow to cool prior to cutting into bite-size pieces. Deglaze the pan by adding the stock and stirring to pick up any browned bits in the pan. Whisk in the butter. Once completely melted, add the sumac and red pepper flakes, and simmer until it reduces by a third, about 3 minutes. Add the shrimp back into the pan along with the sliced spring onion greens and the grilled spring onion pieces. Toss to coat and allow the shrimp to warm through, but do not overcook!
If you can't find sumac, just use the zest of a lemon.