Shabu Shabu Noodle Soup

Name: Shabu-Shabu Noodle Soup
Ingredients: 5 scallions; 2 Tbsp. (heaping) instant dashi powder (preferably Hondashi); ¼ small head of cabbage, cut into 1 pieces; 4 oz. shiitake mushrooms, stems chopped, caps thinly sliced; 2 cups trimmed bean sprouts; 8 oz. shaved beef, cut into 3x2 pieces, patted dry, chilled; 14–16 oz. fresh wheat noodles (such as udon); 2 Tbsp. (or more) soy sauce; 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt, plus more; Lemon wedges (for serving; optional)