Combine the cilantro and garlic on a cutting board, and use the back of a chef's knife to work them into a fine paste; a pinch of coarse salt helps this process. (If you own a mortar and pestle, there's never been a better time to use it.) Transfer the paste to a bowl, and add the avocado. Use a fork to smash the avocado into a mostly smooth—but still slightly chunky—puree. Stir in the onion, jalapeño, lemon juice, and salt. Serve with tortilla chips or warm corn tortillas.