In a large pot, combine the dashi, soy sauce, mirin, and sugar.
Bring to a simmer over medium heat and cook for about 5 minutes, stirring to dissolve the sugar.
If using frozen udon noodles, cook them according to the package instructions.
If using fresh udon noodles, simply rinse them under warm water to separate the strands.
In a separate pot, bring water to a boil and blanch the spinach or bok choy for 1-2 minutes. Remove and set aside.
In the same pot, blanch the julienned carrots for 1-2 minutes and set aside.
If using cooked chicken or beef, slice into thin strips.
Divide the cooked udon noodles among serving bowls.
Pour the hot broth over the noodles, distributing it evenly.
Arrange the cooked protein, mushrooms, carrots, and spinach or bok choy on top of the noodles.
Garnish with sliced green onions, nori strips, tempura flakes, and toasted sesame seeds as desired.
Enjoy your meal!