Lift the greens through several changes of cold water until no grit remains, then drain well. Chop the leaves roughly and any tender stems finely. The weights above are after trimming. If using the optional fenugreek, use it with 250 g spinach rather than 300 g.
Put 300 ml water in the saucepan with the ginger, 12 g chopped garlic, green chillies, turmeric and salt. Bring to a simmer over a medium heat. Add the mustard greens and spinach, plus optional fenugreek, in batches, covering briefly to wilt each addition. Allow about 5 to 10 minutes for all the greens to fit without forcing the lid down.
Cover and cook over a low heat for about 45 to 60 minutes, stirring every 10 to 15 minutes. Keep a little liquid at the bottom; add hot water if it looks dry. Continue until the leaves and tender stems crush easily with a spoon. Older greens may take longer, so use tenderness as the guide.
Remove from the heat and allow vigorous bubbling to stop. If there is a large pool of liquid, simmer uncovered briefly to reduce it before blending. Use short pulses of a stick blender, keeping its head submerged and following its hot food instructions, to make a thick mixture with some leafy texture. Alternatively, mash thoroughly by hand. Remove any remaining tough fibres.
Whisk the maize flour with 60 ml cool water until smooth. Stir gradually into the greens. Return to a low simmer for about 20 minutes, stirring frequently and scraping the bottom. Add small splashes of hot water if the mixture becomes stiff. It should become cohesive and lose any raw flour taste. Continue cooking if it still tastes floury.
Heat the ghee and oil in the frying pan over a medium heat. Add the onion and cook for about 10 to 12 minutes until golden. Add the cumin and stir for 20 to 30 seconds, then add the reserved 8 g sliced garlic. Cook for about 30 to 60 seconds, just until lightly golden, lowering the heat if necessary. Remove from the heat before the garlic browns deeply.
Carefully stir all the tarka, including its fat, into the saag. Simmer gently for 5 minutes. Taste and adjust the salt if needed, and add a little hot water if it is too thick. Serve hot with the optional butter, warmed roti or chapati, sliced onion and pickle.