Preheat your oven to 250°F (120°C). Spread baking soda evenly on a baking sheet and bake for 1 hour, then cool.

In a large mixing bowl, combine all-purpose flour, the cooled baked baking soda, and salt. Gradually add water while mixing until a shaggy dough forms. Knead for about 5 minutes until smooth and rest for 30 minutes.

Let the dough rest in a covered bowl for at least 3 hours at room temperature or up to 12 hours in the fridge.

Divide the dough into portions and roll through a pasta maker to ⅛-inch thickness. Cut into spaghetti-sized strands.

Soak the dried shiitake mushrooms and kombu in water for at least 1 hour.

Broil leeks, onions, garlic, tomatoes, and carrots on a baking sheet for 10-15 minutes.

In a large pot, simmer soaked kombu and shiitake mushrooms in fresh water with broiled vegetables for 1-3 hours. Strain the liquid.

Press and slice tofu into cubes. Fry in a skillet over medium-high heat for 5-7 minutes until golden brown.

Stir soy sauce and mirin into the strained broth. Keep warm.

Bring a pot of salted water to a boil. Cook noodles for about 1 minute until al dente. Drain and rinse under cold water.

Assemble bowls by adding sesame oil, pouring in broth, adding noodles and toppings, and garnish.