Place the chile, unpeeled garlic and onion slice on a pre-heated comal or skillet set over medium heat, or place under the broiler on a sheet pan. Char or roast for about 10 minutes, flipping occasionally so they char evenly, until blackened. Remove from heat and set aside to cool.
When cool enough to handle, peel the garlic and cut the stem from the chile.
Coarsely chop, chile, garlic and onion. Place in bowl or molcajete, along with the lime juice and salt, and mash or mix until pasty. Incorporate the cilantro and avocados, and mash until you reach your desired consistency (I like it chunky). Done!