Heat the a nonstick pan on medium heat. Once hot, add the beef and brown while crumbling into smaller pieces. Remove the beef with a slotted spoon and set aside, leaving the grease in the pan. Add the mushrooms to the pan and saute until they are browned and soft, then add the garlic, ginger, and peanuts; saute for another 2 to 3 minutes.
Add the sesame oil, tahini, soy sauce, vinegar, chili oil (if using) and sugar. Add water as needed to thin out the sauce. Stir well then remove pan from the heat.
Bring a large pot of water to a full boil. Cook noodles following the instructions on the package. Once cooked, drain and rinse under cold water for a few seconds to remove excess starch.
Return the pan to medium heat, toss the sauce with the noodles and beef. Garnish with scallions. Serve with bread.