In a food processor, pulse the cashews until they are just chopped (I like a bit of texture or the Satay will be too creamy). Set aside.
Now add the red onion, garlic, ginger and lemongrass to the processor and process to a rough paste (if you don't have a processor you can finely grate them or pound in a mortar and pestle).
Over a low heat, melt your fat and add the onion mix. Saute for a few minutes until the onion starts to soften.
Then add the chopped cashews, coconut milk, optional chili, sweetener and 125ml (1/2 cup of water).
Simmer for 5-10 minutes or until the sauce thickens.
Stir in the tamari and lime juice and taste it. Adjust the heat (chilli), saltiness (tamari), sweetness (rice syrup or coconut sugar) and sour flavours (lime juice) to your taste, if necessary.
Delicious served one of the ways below and with lime cheeks and fresh coriander leaves.