Here's the best way: I've tried many varities of achar in the US. None seems to come close to my secrets.
The secrets: Tomatillo and fresh green curry leaves (not dried ).
Tomatillo (NOT tomato, it's green wrapped with some green leaves which the Mexican use a lot). It can be sticky a little bit, and if it gets old, it becomes very sour.
You need regular tomato too.
2:1 ratio. two tomato and one tomatillo.If you add more tomatillo, it will be a greeen achar instead and too sour. Make sure the tomatillo is just few if you're making a big size.
Bake them (both of them) in Oven till slighty burned. Tomatillo may some take time depending uopn how ripe they're.
Heat and roast some sesame seeds.
Then heat some oil and when burning hot, add some curry leaves (2-3 will be more than enough). You'll get the aroma right away
Grind the tomato, tomatillo in a grinder, add the roated sesame seeds, a pinch of timbur, some salt and little garlic, RED chillie powder (according to your taste), lots of fresh cilantro and make a mixture of all above.
Add the mixture on the oil with the curry leaves (don't let it burn) and cook for few minutes. Add water if they become too thick
Boom, the best tasting MOMO achar you can get. My mexican friend taught me, but the concept of curry leaves, I added myself of the great smell it gives.