In a large mixing bowl, add the garlic, lime juice, sugar, and olive oil. Season with salt and pepper, to taste, and give it a quick whisk to combine.
Split avocados in half and remove the pit. Spoon out the flesh, put on a cutting board and slice into chunks. Transfer to a serving bowl and add the onion and cilantro leaves. Add the vinaigrette and toss lightly to coat just before serving.