Cook udon noodles according to package directions. One minute before cooking time ends, add snow peas to pot. Drain and set aside.

While noodles are cooking, whisk together peanut butter, rice vinegar, soy sauce, sesame oil, brown sugar, ginger, garlic, and red pepper flakes or sriracha (if using). Whisk in water 1 tablespoon at a time until sauce is smooth but still flavorful. (Different peanut butter brands will produce different consistencies of sauce, so it's hard to be exact about the amount of water needed. The sauce will thin out a little bit once added to the noodles, so don't add too much water or the end result will be bland.)

Add noodles and snow peas back to pot. Stir in peanut sauce. Serve topped with roasted peanuts and additional sriracha, if desired.