Combine the chicken broth, tamari, mirin, sesame oil, ginger slices, and smashed garlic in a large pot. Bring to a simmer over medium heat and let the aromatics infuse the broth for about 10 minutes.
Add the chicken thighs to the simmering broth and cook for 15 to 18 minutes, until the chicken is cooked through and reaches 165°F internally. Remove the chicken to a cutting board and shred it using two forks. Strain the broth through a fine mesh strainer to remove the ginger and garlic, then return the broth to the pot and keep it warm.
While the chicken cooks, bring a separate pot of water to a boil and cook the rice noodles according to the package directions. Drain, rinse with cold water, and set aside.
Prepare the soft-boiled eggs by lowering them into boiling water and cooking for exactly 7 minutes. Transfer immediately to an ice bath, then peel and halve them.
Add the baby spinach and corn kernels to the warm broth and stir for 1 to 2 minutes until the spinach wilts.
Divide the cooked rice noodles between two large bowls. Ladle the hot broth with the spinach and corn over the noodles. Top each bowl with shredded chicken, a halved soft-boiled egg, sliced green onions, nori strips, a drizzle of sriracha, and a sprinkle of sesame seeds. Serve immediately.