3Tbspmentsuyu/tsuyu (noodle soup base)(Mentsuyu I use is 3-times concentrated. If you use non-concentrated Mentsuyu, you might need to add more to achieve the same flavor. You could skip Mentsuyu and increase the soy sauce to a total of 2-3 Tbsp (gradually increase, please), but the final dish will taste slightly different. Here's my homemade mentsuyu recipe.)
Gather all the ingredients. I like using frozen Sanuki udon noodles, which require blanching in boiling water for just 1 minute before using. If you use dried udon noodles, boil them according to the package instruction.
Bring a large pot of water to boil and cook udon noodles.
Slice onion and cut cabbage into 1” (2.5 cm) square pieces. Julienne the carrot into 2” (5 cm) long strips.
Discard the tough shiitake stems and slice the mushroom tops. Thinly slice the top 2” (5 cm) green part of scallions and set aside (for garnish). Cut the rest of the scallion into 2” (5 cm) pieces.
Cut the pork belly slices into 1” (2.5 cm) pieces.
In a frying pan, heat oil over medium-high heat. Add the pork and cook until almost cooked through. Then add onion and cook until translucent and soft.
Add cabbage and carrots and stir fry until coated with oil. Then add shiitake mushrooms and scallion. Stir fry until vegetables are lightly wilted.
Add udon noodles and using tongs, combine well with all the ingredients.
Add freshly ground black pepper, 3 Tbsp mentsuyu, and 1 tsp soy sauce and mix all together. Please adjust the amount of mentsuyu to your liking.
Serve on a plate and sprinkle bonito flakes and green onions on top. Garnish with red pickled ginger on the side if you like.
You can keep the leftover in an airtight container and store in the refrigerator for up to 3-4 days and in the freezer for a month.