Make the pickled cucumbers. Combine rice vinegar, boiling water, sugar, and salt in a medium bowl, stirring until sugar and salt dissolves. Add sliced cucumbers and let sit at room temperature for 30 minutes or in the fridge overnight.
Prepare crumbled tofu topping. Wrap the drained tofu with paper towels and place a weight on top. Let sit for 10 minutes, draining the excess water. Remove paper towels and crumble tofu into small chunks.
Heat 2 tablespoons oil in a large nonstick saute pan over medium heat. Add crumbled tofu and let sit undisturbed for 5 minutes. Season with salt and stir to combine. Continue to cook until golden brown. Remove tofu from pan and set aside.
In the same pan, heat another 2 tablespoons oil. Add chopped mushrooms and saute until browned, about 5 minutes. Add garlic and ginger and cook for 1 more minute. Add back tofu along with Shaoxing wine, soy sauce, hoisin, sugar, and five spice powder. Mix to combine and simmer for 2 minutes. Season with salt and pepper and keep warm.
Make sauce for noodles. Whisk together boiling water with chili oil, sesame paste, soy sauce, sugar, and Sichuan peppercorns until well combined.
Cook udon noodles according to directions on package. Drain and immediately add as much as sauce as you like, stirring to combine.
Portion noodles into bowls and top with crumbled tofu. Garnish with pickled cucumber, chopped peanuts, and cilantro. Serve immediately.
Notes
The pickled cucumbers and sauce for the noodles can be prepared the day before.