Remember to start rice and other sides.
Mix the marinade. If using meat, cut into thin slices, about 1/8" thick. If using tofu more like 3/8" thick, since it doesn't hold together as well. Put the protein in the marinade. Longer is better, though since it's cut thin even ~10min is ok.
Cut up the onion and cook it, set aside. If you're putting in a pepper chop it up and also set it aside. If you want to use more veggies you can, but either use less protein or mix a larger quantity of marinade.
Pick the protein out of the marinade and cook in a skillet at medium-high heat, with a bit of cooking oil. You want the pieces to get dark brown, but not to burn. If using tofu be gentle with it. Don't crowd it. Set aside pieces as they're finished cooking.
Right before serving, put everything back in the pan, add the cooked onion, optionally add peppers, and add the rest of the marinade. Add the brown sugar and the sesame seeds. Let the marinade come to a boil, for safety. Stir well, though with tofu continue being gentle.
Optionally put scallions on it.