Prepare the Ramen Broth: Add all ramen broth ingredients (chicken broth, light soy sauce, mirin, ginger, garlic, green onions, lemongrass, and red pepper flakes) to a large Dutch oven or stockpot. Bring to a boil, then reduce heat to medium to medium-low and simmer for 15-20 minutes.
Cook the Chicken: While the broth is simmering, season the chicken thighs with black pepper. Heat vegetable oil in a skillet over medium-high heat, and cook the chicken thighs for 4-5 minutes per side, until golden brown.
Make the Glaze: Whisk together the remaining glazed chicken ingredients (light soy sauce, dark soy sauce, brown sugar, minced ginger, and minced garlic), then pour over the chicken thighs in the skillet. Cook the mixture down until syrupy and thick, turning the chicken occasionally to glaze on both sides. This process usually takes about 5 minutes.
Slice the Chicken: Remove the chicken thighs to a cutting board to rest for a few minutes, then slice thinly.
Strain the Broth: Once the ramen broth is done simmering, place a very large mixing bowl in your sink and add a colander to that. Pour the broth over the colander, straining the solids from your broth. Return the strained broth to the pot and place it back on the stove.
Cook the Vegetables: Add the sliced mushrooms to the broth and cook for 2-3 minutes over medium heat. Add the baby spinach and sesame oil, and cook for another 3 minutes.
Cook the Ramen Noodles: Add water to another pot and boil the ramen noodles for 1 minute less than the package directions specify. Drain the noodles and divide them among 4 bowls.
Assemble the Ramen: Pour the hot ramen broth with mushrooms and spinach over the noodles in each bowl. Top with sliced chicken and garnish with green onions. Add any additional toppings you desire, and serve hot.