Bring 2 litres of water to a boil in a large pot for the noodles.
In a separate saucepan, combine the vegan dashi, soy sauce, mirin, salt and sugar. Bring to a gentle simmer, stir until the sugar dissolves, then keep warm on low heat.
Slice the spring onions into thin rounds and set aside.
Cut a 1-inch thick slab from your tofu block and slice it diagonally into two triangles. Dust the tofu lightly with cornflour until coated on all sides.
Heat 1–2 cm of neutral oil in a small frying pan until it reaches about 180°C. Fry the tofu until golden and crisp on both sides, then drain on a rack or paper towel. Sprinkle with salt while still hot.
For fresh udon, add the noodles to the boiling water and gently tease them apart with chopsticks until loose and bouncy. For dried udon, cook until just shy of the package instructions, then drain and rinse under cold water to remove excess starch. Briefly reheat in hot water before serving.
Divide the noodles between serving bowls, ladle over the hot broth, and top with crispy tofu. Garnish with spring onions and a sprinkle of shichimi togarashi if you like spice.