Heat the neutral oil in a large pot or Dutch oven over medium-high heat. Add the thinly sliced pork belly or chicken and cook until lightly browned and cooked through, about 3-4 minutes.

Add the sliced onion to the pot and sauté until it softens and becomes translucent, about 3-4 minutes. Stir in the minced garlic and grated ginger and cook for another minute until fragrant.

Pour in the dashi stock (or chicken broth) and bring the mixture to a simmer. Use a spoon or a fine-mesh skimmer to remove any scum that rises to the surface for a cleaner broth.

Turn the heat down to low. Break the Japanese curry roux blocks into smaller pieces and add them to the pot. Stir gently and continuously until the roux has completely dissolved into the broth.

Once the roux is dissolved, stir in the soy sauce, mirin, and sugar. Let the soup simmer gently for 5 minutes to allow the flavors to meld together. Do not let it boil rapidly.

In a small bowl, mix the potato starch (or cornstarch) with 2 tablespoons of cold water to create a slurry. While stirring the soup, slowly pour in the slurry to thicken the broth. Continue to simmer for another 1-2 minutes until the broth has slightly thickened. Keep warm over low heat.

While the curry soup is simmering, bring a separate saucepan of water to a boil. Cook the frozen udon noodles according to the package instructions (usually 1-3 minutes). Drain them well immediately to prevent them from getting mushy.

Divide the freshly cooked and drained udon noodles between two large serving bowls.

Ladle the hot, rich curry soup generously over the noodles in each bowl, making sure to include the pork/chicken and onions.

Garnish with your desired toppings. A halved soft-boiled egg, a generous sprinkle of thinly sliced green onions, and a dash of shichimi togarashi for a little extra spice and flavor are classic choices. Serve immediately and enjoy!