Beef & Broccoli Stir Fry

This was originally a recipe I found on Skinnytaste for chicken and broccoli stir fry. Other attempts I've made at stir-fry haven't been great, but this one really worked beautifully. I substituted thinly-sliced flank steak to mimic the Chinese takeout staple. For a more vegetal take, just add thinly-cut carrots, peppers, or snow peas.

• 1 lb flank steak (sliced against the bias into 1/8" strips)
• 1 tbsp garlic (minced)
• 2 c brocolli florets (raw)
• 1/2 c shiitake mushrooms (sliced)
• grape seed oil
• sesame seeds

Stir-fry sauce:

• 1/2 c beef stock
• 1/4 c soy sauce (low sodium)
• 1 1/2 tbsp corn starch
• 1/4 tsp white pepper
• 1 tbsp grated ginger
• 2 tbsp mirin
• 1 tbsp brown sugar
• 2 tsp sesame oil