1. Place beef in freezer for 20 minutes. Remove from freezer; cut across the grain into very thin slices. Place beef in a medium bowl. Add soy sauce and next 5 ingredients (through garlic); toss well to combine. Marinate in refrigerator for 1 hour.
2. Soak noodles in cold water for 30 minutes; drain (noodles will not be tender). Cut noodles with kitchen scissors 2 or 3 times.
3. Trim roots from enoki mushrooms; separate mushrooms into several large clumps. Cut carrots crosswise into thirds; cut each piece lengthwise into 1/8-inch-thick slices.
4. Heat a Dutch oven over high heat. Add beef mixture to pan; stir-fry 2 minutes. Arrange noodles over beef mixture; add stock. Reduce heat to medium-high. Top noodles with enoki, carrots, shiitake, tofu, jalapeño, and onions. Bring mixture just to a simmer; simmer gently for 5 minutes or until noodles are tender. Stir in salt.