Cut avocados lengthwise, around pit. Separate the halves and cut lengthwise again along the pit half, allowing the pit to come out easily. Cut the non-pit half in half as well, and peel off the skin from all four quarters.
Place the peeled avocado into a medium bowl and juice the lime over them. Let sit while you cut onions, garlic, cilantro, and tomato.
Add salt, cumin and cayenne to the avocados. With a tomato masher, mash the avocado mixture to the consistency you desire (smoother or chunkier). Then fold in the onion, garlic, tomato and cilantro. Let sit at room temperature for at least 10 minutes. Taste. Add more salt or lime depending on your taste and/or avocado ripeness.
Combine all ingredients into a salad dressing shaker and shake well.
Toast your bread to your liking. Drizzle about 1/2 tsp of Balsamic Vinaigrette on the toast. Spoon guacamole onto the toast and drizzle more vinaigrette on top (to your liking). Top with arugula or your choice of lettuce.