From My Bowl

Prep: Bring a large pot of water to a boil and cook the udon noodles according to the package instructions; dried noodles will take around 8-12 minutes while fresh noodles take 1-2 minutes.
Make the Sauce: In a small bowl combine the peanut butter, garlic, tamari, vinegar, sugar, sesame oil, and chili garlic sauce. Whisk together until a thick paste forms, then add 2 tablespoons of hot water and whisk until a thick sauce forms and the sugar has dissolved. Add more water if necessary, but keep in mind that the sauce will thin out more when combined with the noodles.
Combine: Drain the noodles well and return to the pot or a shallow pan. Pour the peanut sauce over the noodles and use tongs to mix well, until every noodle is coated. The residual heat left from cooking the noodles usually warms them up enough for me, but you can also gently heat the noodles over medium low heat if desired.
Serve: Top with green onion, crushed peanuts, and/or chili oil and serve warm. Leftovers will keep in the fridge for up to 4 days and are best reheated on the stovetop or in the microwave with a splash of water to help thin the sauce out again.