For the spice mix:
Place the black peppercorns into a mortar and pestle and grind. Add in the Sichuan peppercorns and papaya seeds, if using, and grind. Transfer the spice mix to a container.
For the steak:
Preheat the oven to 400 degrees F. Heat a cast iron pan or skillet over high heat.
Press the Sichuan peppercorn spice mix into all of the sides of the steaks, coating entirely and ensuring the spices adhere to the meat. Add the vegetable oil to the cast iron pan, and heat. Season the steaks with salt, to taste.
Once the pan is smoking hot, add the steaks. Sear the steaks for 2 minutes or until golden. Do not move. Turn the steaks over, and transfer the pan to the preheated oven for 5 minutes, or until desired degree of doneness. Remove the pan from the oven, turn the steaks over, and set them aside. Allow the meat to rest for 10 minutes so the juices redistribute, before serving.
For the onions:
Heat a grill over medium-high heat and lightly grease the grates with oil.
Toss the green onions in the vegetable oil, and season them with salt and pepper to taste.
Grill the green onions for 1 minute. Flip the onions over, and continue cooking for 1 minute, or until onions have softened and are slightly charred.