In the pot, cook rice noodles according to package instructions. Strain and rinse with water then set aside.
In the same pot, boil water and black your carrots and bokchoy, if using, about 2 to 3 minutes. Once cooked, strain, dispose the water and set aside.
Pat chicken fillets dry. Season with salt and pepper.
In the same pot, heat the pot until the water has dissolved. Add unsalted butter until melted. Sear chicken in medium-high heat until golden brown, about 3 to 4 minutes. Turn to the other side and sear again. Lower heat until chicken is cooked through (thickest part registers 165F/ 74C). Set aside or optionally, you can slice them into strips.
In the same pot (without washing), add the onions and cook until translucent, about 3 minutes. Then add the garlic until aromatic, about 2 minutes.
Pour your chicken stock to the same pot. Add in the red miso paste and your shabu-shabu balls until cooked through and miso paste is fully dissolved. Season with salt and pepper to taste. While waiting, assemble your cooked ingredients in a bowl, starting with the cooked noodles. Add your toppings to the top of the noodles. Ladle in your hot soup and end by placing the chicken fillet on top. Garnish with scallion or leek.