Set up your workspace with a large pot, a sharp knife, and a cutting board. Slice the onion and garlic thinly; set aside.
Preheat your oven to 200°C (392°F). Toss the sliced onion, garlic, and ginger with a teaspoon of sesame oil, then spread them on a baking sheet. Roast for about 20 minutes until fragrant and slightly charred at the edges.
Pour a tablespoon of sesame oil into your large pot over medium heat. Add the roasted aromatics along with the sliced ginger, and cook for about 3-4 minutes until the kitchen fills with a warm, nutty aroma.
Stir in the miso paste, dried shiitake mushrooms, and torn nori pieces. Cook for another 2 minutes, pressing the miso into the hot oil to release its rich flavor.
Pour in the water, then bring the mixture to a boil over high heat. Once bubbling, reduce the heat to low and let it simmer gently for about 45 minutes, stirring occasionally and skimming any foam that appears.
Carefully strain the broth through a fine-mesh strainer into a clean pot, discarding the solids. Taste and adjust saltiness with soy sauce if desired.
Turn off the heat. Stir in the remaining teaspoon of toasted sesame oil, a splash of rice vinegar, and a squeeze of lemon if you like a brighter flavor. Keep the broth warm.
Cook your vegan noodles according to the package instructions until just al dente—usually about 3-4 minutes. Drain and set aside.
Divide the cooked noodles into four bowls. Ladle the hot broth over the noodles, allowing them to soak up the savory flavors.
Garnish each bowl with sliced green onions and additional toppings like bamboo shoots or vegetables if you like. Serve immediately while steaming hot, with chopsticks and a spoon.
