In a large bowl, whisk together peanut butter, soy sauce, brown sugar, Sriracha and 2 tablespoons water; set aside.
Heat vegetable oil in a large saucepan over medium high heat. Add beef and cook until browned, about 3-4 minutes; season with salt and pepper, to taste. Stir in peanut butter mixture until well combined, about 1-2 minutes.
Serve immediately, garnished with green onion and peanuts, if desired.
