Cook the noodles according to the package directions. Drain, transfer to a large bowl, and cover with aluminum foil or plastic wrap to keep warm.
Meanwhile, in a medium saucepan, whisk together the sauce ingredients until well blended and set the pan over medium heat. Bring to a simmer and let simmer for 10 minutes.
Heat the peanut oil in a large skillet over medium-high heat. Add the ginger and garlic and cook, stirring, for 2 minutes. Add the bell pepper and cook, stirring, for 2 minutes. Add the beef and cook, stirring, 2 minutes. Add 1 cup of the peanut sauce and simmer until the beef is cooked through, about 5 minutes.
Transfer the beef mixture to the noodles and toss to combine, and then serve on individual plates topped with the peanuts and scallions. The leftover peanut sauce will keep in the refrigerator up to 4 days or in the freezer up to 3 months; thaw completely in the refrigerator or microwave for about 3 minutes on LOW before using.