Heat oil on a pan. Fry the onion until translucent. Add the ginger-garlic paste and the red chilli paste. Fry until the raw smell goes away, for about a minute or more.
Add the coconut milk and lemon grass and let the mixture come to a rolling boil.
Add the basil leaves. Turn off the heat and keep it covered so that the basil flavours infuses with the mix.
Meanwhile, heat the momos in the microwave.
Pour the red curry sauce over the momos and serve.