Combine the mirin, tamari, garlic and ginger and pour over the carrot and sweet potato. Set aside to marinade for about 30 minutes.
Pour approx 5 cm boiling water into a large pan and bring to the boil. Place a steamer basket, inside the pan and a heatproof bowl on top ( you could also use a parcel made from foil to steam the veggies in).Add the sweet potato to the bowl, cover and steam for 7 minutes. Remove the cover and add the carrots to the bowl, cover and steam for a further 5 minutes. Next, add the add the asparagus, zucchini, mushrooms, spring onions and tofu. Stir gently and cover again to steam for a further 4 minutes. Remove from the heat and serve.
Note: Carrots are believed to act as toxic mops, absorbing toxins from the soil while they grow. It's best to buy organic carrots at all time, available from most supermarkets.