I feel shallots compliment the avocado more than onions. Also, mashing the shallots, jalapeno and garlic with rock salt brings out their juices and oils and it infuses the guacamole nicely.
I've been asked how you can tell if an avocado is ripe. I say make a fist and press into the flesh between your thumb and index finger. That's what an avocado should feel like if it's perfectly ripe. Any harder and it needs a few days to ripen. If it's really hard, it may never ripen.
NOTE: You can increase or decrease the amount of heat by adding or reducing the amount of jalapeno.
TIP: To easily remove the pit, give it a whack with the sharp end of a large knife and twist. It will pop right out!