Preheat your oven to 425°F / 215°C. On a baking sheet, toss the diced sweet potato with olive oil and grated ginger.
Roast for 15 to 20 minutes, until golden brown and tender.
While the sweet potato roasts, cook the udon noodles according to package directions.
Drain, rinse with cold water to prevent sticking, and set aside.
In a large pot, combine the broth and 4 cups / 950 ml of water.
Bring to a simmer over medium heat.
In a small bowl, whisk the miso with a few tablespoons of the warm broth until smooth, then stir it back into the pot along with the soy sauce or tamari.
Stir in the spinach, wakame, dulse, and half of the sliced scallions.
Add the roasted sweet potato and diced tofu, and let it all simmer for about 1 minute to heat through and wilt the spinach.
To serve, divide the cooked noodles among 5 bowls. Ladle the hot soup over the noodles.
Top with fresh enoki mushrooms and the remaining scallions.
Drizzle with sesame oil and add a spoonful of sambal oelek for a touch of heat, if desired.