Wrap tofu in paper towels and place heavy books on top for 30 minutes to dry out tofu.

Preheat the oven to 400 degrees F (200 degrees C). Grease a baking sheet.

Chop pressed tofu into 1-inch cubes and place on the prepared baking sheet.

Bake in the preheated oven for 10 minutes. Turn tofu and bake an additional 10 minutes until crispy.

While tofu bakes, bring a large pot of lightly salted water to a boil. Cook udon in boiling water according to package directions, stirring occasionally, until noodles are tender yet firm to the bite, 10 to 12 minutes. Drain.

Heat oil in a large pot over medium heat. Cook serrano pepper and ginger until softened, 2 to 3 minutes. Add vegetable broth, carrots, green onions, bok choy, shiitake mushrooms, sesame oil, soy sauce, and baked tofu. Bring to a boil, reduce heat, and simmer for 15 minutes, stirring occasionally. Add cooked udon noodles and simmer 5 more minutes.