This is the noodle version of the dish you've probably eaten at P.F. Chang's and wondered why yours never comes out as crispy at home. The answer is always the same three things: the beef is too thick, the pan isn't hot enough, or you crowded it.
Fix those three and this dish is fifteen minutes of work. Start by slicing the flank steak as thin as you can manage, always against the grain. Frozen for twenty minutes first makes this easier. Thin slices cook fast enough to crisp on the outside without turning gray and sad on the inside, which is the whole trade-off with cheap steak.
Coat the slices in a light dusting of cornstarch. Not a batter. Just enough to dry the surface so it takes real color in the pan. The cornstarch also thickens the sauce later without you doing anything else, which is why you don't slurry it separately.
The sauce is a Beijing-style sweet-salty-tangy balance: soy for the savory base, beef bouillon for depth, brown sugar for the caramel note that matches the crisp beef edges, rice vinegar for the acid that keeps it from being cloying. Chili flakes take it from suggestion to serious. If you want it bolder, double the bouillon, not the soy.
Cook the rice noodles directly in the sauce at the end. That's how they pick up color and flavor instead of arriving on the plate white and blank. The whole pan should be glossy by the time the noodles are tender, with just enough liquid left to coat every strand. If it's drying out too fast, splash in water by the tablespoon.