To quick pickle the carrots and radish, just boil the rice vinegar, then pour it over shredded carrots and radish. Fill to the top with a little water, then season with salt and pepper and a pinch of sugar and let it sit an hour or 2.
To start the meatballs, add the butter to a frying pan over medium heat and add the diced onion. Cook about 15 minutes to soften and start browning. Allow to cool for 5 minutes
Mix all the meatball ingredients including the cooked onions in a large bowl. Line them up on a baking sheet and broil about 7 minutes, flip, broil about 5 minutes until cooked through.
To make the sauce for the meatballs, in a pan heat the sugar and water until the sugar dissolves.Once dissolved, stop stirring. Bring to a boil and when it starts to brown, kill the heat and add the meatballs and a squirt of sriracha and fish sauce. Stir and coat the meatballs and allow them to heat through.
Slice the bread into three sandwich sized rolls. Cut and toast the roll, top with cucumbers, then the meatballs, then the carrots, and basil. Serve!