In a medium bowl, stir together the beef broth, soy sauce, hoisin sauce, sesame oil and sugar. Set aside.
Cut the flank steak into thin strips, against the grain of the meat.
In another bowl, mix together the beef strips, the cornstarch and ¼ cup of the sauce you made in step #1 (reserve the rest of the sauce!) Cover the bowl with plastic wrap and set in the fridge for 30 minutes.
While the beef is marinating in the fridge, cook the bean thread noodles according to package directions. They cook very, very quickly!
Cut up all the vegetables.
Heat 1 teaspoon of canola oil over medium-high heat in a large skillet.
When the skillet is hot, add the garlic and the ginger. Cook over medium-high heat for 30 seconds.
Add the carrots, sliced bell pepper, and white parts of the green onions to the skillet. Cook on medium-high heat until the vegetables are tender-crisp.
Turn the heat down to medium, add the broccoli, the kale, and the green parts of the green onions. Put the lid on the skillet and steam the veggies for about 3 minutes, until the broccoli is tender-crisp and the kale starts wilting.
Remove the cooked vegetables to a separate plate and set aside.
Spritz the skillet with nonstick spray and crank the heat back up to medium-high. Cook the beef until no longer pink. (You might need to do this in two batches-- we're trying to get a nice sear on our beef. If you fill the skillet with too much beef, the meat will just stew in the pan and you won't get that crispy sear on it.)
Leave the beef alone for the first minute of cooking so it can develop a nice seared crust on the bottom. After that, stir-fry the beef until all the pink is gone.
Add the vegetables, the noodles, and the reserved sauce back into the skillet with the beef. Toss it all together.