In a small bowl, whisk together the soy sauce sesame oil, and mirin. Set aside.
In a large cast-iron skillet, heat 1 tbsp oil. Season the shrimp with ½ tsp salt and cook for 1-2 minutes on each side until pink being careful not to overcook (the shrimp will curl up when it is cooked too long)
Remove the shrimp form the pan and set aside.
Add 1 tsp oil in the same pan and add the onion, sautéing until translucent. Add the garlic and scallions and sauté 2 minutes more.
Push the onion and garlic to the outside of the pan and add the last 1 tsp oil to the center of the pan. Add the eggs in the middle and scramble them.
Now add the butter, cooked rice, peas and reserved sauce, tossing to combine,
Lastly, add the shrimp and mix to heat through.
Garnish with sliced scallions and sesame seeds.