Scoop the avocado flesh into a medium bowl. Add the lime juice and mash with a fork until mostly smooth but with a few small chunks for texture.
Add the Greek yogurt (or sour cream), olive oil, minced garlic, salt, and cumin if using. Stir vigorously with the fork until well combined and creamy.
Gradually add water, one tablespoon at a time, stirring after each addition, until the dressing reaches your desired consistency. It should be thick enough to coat the back of a spoon but still pourable.
Taste and adjust salt or lime juice as needed. Transfer to a leak-proof jar or container. Press a piece of plastic wrap directly onto the surface before sealing to reduce browning.
Store in a cooler with ice packs. Shake or stir before using. The dressing will keep for up to 3 days.