If using the plain flour, sprinkle on top of the onion and stir through, creating a bit of a paste. Cook off the flour, continuing to stir but not brown for 1-2 minutes. Add half a cup of the water and whisk through briskly. Whisk until the gravy thickens.
If using the gluten free flour, stir to dissolve first in ½ cup of the water. Pour this into the onions and cook off the flour, continuing to stir but not brown for 1-2 minutes.