If you don't have a molcajete, try to replicate the texture to get as close to that authentic taste as possible. For that, do the following:
1. Grind the garlic, onion and chiles into a paste using a mortar and pestle.
2. Using a blender, a hand blender or a food processor, pulse the tomatillos until they are blended just enough to have some chunks left. I find that a hand blender works the best for this task, followed by a food processor, and then a regular blender.
3. Now, mix all of the ingredients in a non-reactive bowl with a spoon and serve.