In a medium saucepan combine soy sauce, rice vinegar, sesame oil, minced ginger and brown sugar. Cook over medium high heat, whisking occasionally. Add cornstarch slurry and continue cooking until slightly thickened.
Meanwhile, heat olive oil in wok over medium high heat until shimmering. Add garlic and saute for 1-2 minutes. Add onion and saute 2 minutes more.
Add broccoli and toss to coat. Add shrimp, tossing to coat. Stir fry for 3-4 minutes, or until shrimp is opaque and the tails have curled up.
Add in thickened sauce from the saucepan and stir fry an additional 2-3 minutes.
Notes
It's totally okay to use frozen shrimp for this recipe as most shrimp come frozen, however, you will need to thaw it first. To thaw them quickly, transfer the shrimp to a large ziptop bag and squeeze out the air. Submerge the bag in a bowl of cold water (do not use hot water) for around 10-20 minutes. Pat the shrimp dry and continue with the recipe.
I would not recommend precooked shrimp for this recipe.
Two crowns of broccoli should equal around 2 cups of broccoli florets. You can use frozen broccoli, but allow it to thaw and pat it dry before beginning.