In a large skillet over a low heat, gently fry the onions and leeks in the oil, stirring occasionally, until they start to caramelise. (This should take around 30 minutes.)
Add the roast garlic to the onions and leeks, mash it a little to break it up, and cook for another minute or so.
Stir in the coconut aminos, and allow to evaporate.
Turn up the heat, add the mushrooms, paprika, shiitake powder, and sugar. Cook for another 5 mins, stirring all the time.
Add the stock.
Mix the cornflour with the water to make a paste, and add to the skillet. Keep stirring for a couple of minutes, until the gravy has thickened and just begins to boil.
Reduce the heat to low, cover, and simmer for around 20 minutes.
Remove from the heat, and blend until smooth.
Taste, and season.
If not serving immediately, this mushroom gravy can be stored in an airtight jar in the 'fridge for a couple of weeks. It can also be frozen.