Slice the beef: Trim excess fat and thinly slice against the grain. Thinner slices cook faster and turn out more tender.
Make the sauce: In a bowl, whisk 3/4 cup low-sodium soy sauce, 1 cup beef broth, 1/3 cup brown sugar, 1 tablespoon sesame oil, 1 tablespoon rice vinegar, 4–5 minced garlic cloves, 1–2 tablespoons grated ginger, and a pinch of red pepper flakes.
(Optional) Quick sear: For deeper flavor, heat a little oil in a skillet and quickly sear the beef in batches, 1–2 minutes per side.
Don’t cook through—just brown the edges.
Load the crockpot: Add the sliced beef to the slow cooker. Pour the sauce over the top and stir to coat.
Cook low and slow: Cover and cook on Low for 3–4 hours or on High for 1.5–2 hours, until the beef is tender.
Thicken the sauce: Whisk 2 tablespoons cornstarch with 3 tablespoons cold water. Stir the slurry into the slow cooker.
Switch to High and cook for 15–20 minutes until the sauce thickens slightly.
Add broccoli: Stir in 4 cups broccoli florets. Cover and cook on High for 15–25 minutes, until crisp-tender. For frozen broccoli, thaw first and cook just 10–15 minutes to avoid mushiness.
Taste and adjust: Add a splash of soy sauce for salt, a dash of vinegar for brightness, or a pinch more sugar if you want a sweeter finish.
Serve: Spoon over warm rice or noodles.
Garnish with sliced green onions and sesame seeds.