Add the avocado, sour cream, lime juice, olive oil, garlic, cilantro, diced onion, cumin, and salt to a food processor.
Blend until completely smooth and creamy, about 20–30 seconds.
Scrape down the sides, then add 2 tablespoons of water and blend again until fully combined. Taste and adjust salt as needed. Add the remaining tablespoon of water, if needed, to reach the desired consistency.
Serve immediately with tortilla chips.