Prep: Preheat oven to 375℉. Spray a casserole dish with non-stick cooking spray.
Combine: Put 1 ½ cups long grain white rice, 2 cups chicken broth, 1 tablespoon butter, 2 teaspoons rice vinegar, & ½ teaspoon kosher salt in the prepared casserole dish. Mix to combine.
Bake: Place 24 frozen dumplings (potstickers) & 2 cups broccolini on top of the rice mixture. Cover with aluminum foil and bake for 50 minutes in the preheated oven.
Make Sauce & Spicy Mayo: While it bakes, put all of the sauce ingredients in a mason jar, cover, and shake to combine OR add them all to a small bowl and whisk to combine. Mix up the spicy mayo, if you plan to use when serving.
Serve: Drizzle the top of the casserole with the sauce and spicy mayo. Enjoy with any other toppings, like green onions or sesame seeds!