Whisk together beef broth, soy sauce, brown sugar, oil and garlic in crockpot. Add slices of beef and toss to coat.
Cook on "low" (around 80C) for 6 hours. Note: how thin you cut the beef will affect cutting time; I cut them into maybe 1 inch squares about 1/2 inch thick and 6 hours was too long cooking time.
When crockpot cooking is done, whisk together some cooking liquid and cornstarch in a small bowl to make a slurry. Add it back to the crockpot and stir well. Add frozen brocolli and cook again on low for an additional 30 minutes.
Note on ingredients: I found the sauce didn't thicken up much; the flavor of the sauce was good, but it was runny so I will try less beef broth next time.