Bring a large saucepan of salted water to a boil.
While the water heats, in a blender combine the peanut butter, ginger, vinegar, soy sauce, water, garlic and black pepper. Blend until smooth. The peanut sauce should be thick, but pourable. If the sauce is too thick, add a couple tablespoons of additional water and blend again. Taste, then add hot sauce to your desired heat preference, blending again. Set aside.
In a large skillet over medium-high, heat 2 tablespoons of the oil. Add the onion and peppers, then cook until just tender, about 5 minutes. Transfer to a plate and set aside. Reserve the skillet.
Add the udon noodles to the boiling water and cook for 2 minutes, or until just barely cooked. The noodles should still be a bit firm. While the noodles cook, return the skillet to medium-high heat and add the remaining 2 tablespoons of sesame oil. Drain the noodles well and add them to the hot skillet
Add the shrimp to the skillet and continue to cook, stirring constantly, until the shrimp are just pink, about 5 minutes. Return the onion and peppers to the skillet and toss until heated. Remove the skillet from the heat, then pour in a liberal amount of peanut sauce (use as much or as little as you prefer), using tongs to toss the ingredients until well coated. Top with the peanuts and serve.
