Remove any discoloured or coarse leaves from the cabbage, then halve it. Remove the hard central ribs and discard them, then shred the leaves. If using purple sprouting broccoli, halve any thicker stems lengthways.
Heat the oil in a wok. Stir-fry the broccoli for 1 min, then add the garlic and cabbage and cook until the garlic is a pale gold colour. Quickly add the spinach and fish sauce and turn the veg over – the moisture should come out of the spinach and boil off quickly. Add the sugar and toss the vegetables again, then add a little more fish sauce, if you like.