Combine 1000 g roughly chopped onion, the whole spices for boiling, and 250 ml water in a heavy saucepan or pot, then place over high heat and bring to a boil.
Turn the heat down to medium and gently cook for 20 minutes, watching for the liquid to cook away completely and the onions to soften into translucency.
Empty the contents of the pan into a blender and blitz thoroughly until completely smooth. Store in an airtight container in the fridge for up to 24 hours.
Use 80% of the weight, finely shredded and browned slowly with a pinch of asafoetida. Replicates the bulk and natural sweetness in gravies without allium.