In a large pot, season the boneless, skinless chicken breasts with salt and pepper. Heat 1 tablespoon of sesame oil over medium heat and add the chicken. Sauté for 5-7 minutes per side until golden brown and fully cooked, reaching an internal temperature of 165°F. Remove the chicken, let it rest for a few minutes, then slice into tender pieces.

In the same pot, add more sesame oil if needed and toss in 3 cloves of minced garlic and 1 tablespoon of freshly grated ginger. Sauté over medium heat for 1-2 minutes until fragrant.

Pour in 4 cups of chicken broth, bringing it to a gentle simmer. Stir in 3 tablespoons of soy sauce and 1 tablespoon of miso paste. Allow the broth to simmer gently for about 5 minutes.

Add instant ramen noodles directly to the simmering broth and cook according to package instructions, usually about 3-4 minutes.

Once the noodles are almost done, add 1 cup of sliced shiitake mushrooms and 2 cups of baby spinach to the pot, cooking for an additional 2 minutes.

Return the sliced chicken to the pot and warm it for an extra minute. Ladle into bowls, garnishing with sliced green onions, soft-boiled eggs if desired, and a sprinkle of toasted sesame seeds.