Heat the olive oil in a large pot or Dutch oven over medium heat. Add the finely chopped yellow onion and sauté for 5-7 minutes, stirring occasionally, until softened and translucent. Be careful not to brown the onions too much, as this can affect the soup's delicate flavor.

Add the minced garlic and grated fresh ginger to the pot. Cook for another 1-2 minutes, stirring constantly, until fragrant. This step blooms the aromatics, releasing their full flavor and aroma into the oil.

Add the prepared parsnips and carrots to the pot. Stir them around to coat them lightly with the oil and aromatics. Cook for about 5 minutes, stirring occasionally, to slightly soften the vegetables.

Pour in the vegetable broth and water. Bring the mixture to a boil over medium-high heat. Once boiling, reduce the heat to low, cover the pot, and let it simmer for 20-25 minutes, or until the root vegetables are tender when pierced with a fork.

While the vegetables are simmering, prepare the miso mixture. In a small bowl, whisk together the white miso paste with about 250ml (1 cup) of the hot broth from the pot. This is crucial to prevent the miso from clumping and to ensure it dissolves smoothly into the soup. Using hot liquid helps to temper the miso and preserve its beneficial probiotics.

Once the vegetables are tender, stir the miso mixture into the soup. Add the soy sauce or tamari for an extra layer of umami. Stir well to combine everything. Gently heat the soup for another 2-3 minutes over low heat, ensuring it does not boil.

Taste the soup and adjust seasoning if needed. You can add more miso paste for a saltier, more intense flavor, or a splash more soy sauce. Stir in the toasted sesame oil for a final aromatic touch.

Ladle the hot soup into bowls. Garnish generously with thinly sliced scallions and toasted sesame seeds, if using. Serve immediately and enjoy the warming comfort.