Mix 1 tablespoon soy sauce and all the brown sugar in a medium bowl. Add beef; toss to coat. Marinate at room temperature for 20 minutes. Steam broccoli until crisp-tender, 3 to 4 minutes. Cool under cold running water. Drain.
Heat oil in a large non-stick frying pan over a high heat. Add meat. Stir-fry just until pink, 2 minutes. Remove from pan.
Add mushrooms, capsicum, snow peas, spring onion, garlic, ginger and chilli flakes to pan. Stir-fry until snow peas are crisp-tender, 3 to 4 minutes.
Combine the stock, remaining soy sauce, the vinegar and cornflour in a small bowl until smooth. Add to pan. Bring to the boil (mixture will be thick). Add broccoli. Cook just until heated through, about 2 minutes.
Drain beef. Add to pan. Heat through, about 30 seconds. Serve immediately.
A beef stir-fry is an adaptable dish that is easily varied to make use of seasonal vegetables and a range of seasonings. Try a combination of sugar-snap peas, asparagus spears and radishes with onions and lemon zest. Go for a mix of red, yellow and green capsicums with fresh basil and garlic. Green or butter beans, baby corn and zucchini with spring onions and dill taste good, too.