Char the garlic, onion, tomato and jalapenos on a bbq, oven or dry pan on the stove at medium - high heat (make sure it really gets some good dark char marks on it
Once all ingredients are charred, take the skin off the garlic and place in a blender along with the onion, tomato, 1 jalapeno, cilantro and about 1 tsp of salt
Blend till smooth (If there isn't enough liquid in the blender to blend squeeze in the juice of one lime)
Taste for spice and salt (If it needs more spice, add the second jalapeno) It should be quite spicy at this point but once mixed with the avocados it will mellow out
Mash avocados in a medium bowl
Add the mixture from the blender into the bowl and mix well into the mashed avocados
Add the juice of the remaining lime and adjust salt
Store in the fridge covered for at least 1 hour before serving for the flavours to develop
Notes
If you want to make it a chunky guacamole add some chopped tomatoes, onions and cilantro just before serving.