1. Combine all stir fry sauce ingredients together in a small bowl. Then pour half into an airtight jar and store in the fridge.
2. Slice beef into strips. Then heat the oil in wok or large deep skillet on medium-high heat and fry beef until slightly golden, then put aside in a bowl.
3. Using the same wok, heat half the oil on medium-high heat. Add chopped onion and mushrooms and stir fry 2 minutes.
4. Add chopped capsicum, cauliflower and broccoli and stir fry for 5-7 minutes or until vegetables are tender-crisp.
5. Add beef, sauce and spinach, and stir fry for another 2 minutes until spinach has wilted and sauce has thickened.