Start by toasting your bread (optional, but highly recommended) to your likeness.
While the bread is toasting, let's make the herb mayo.
Immersion Blender: In a mason jar or large cup, add in all of the mayo ingredients except the herb. Insert the immersion blender, making sure the egg yolk is under the blade of the blender, and blend for 40 seconds to one minute or until thick and light in color.
Hand Method: In a bowl, beat the egg with salt and lemon juice until light and fluffy. Slowly drizzle in the oil while continuing to whisk, until the the mixture is thick and creamy. Whisk in the vinegar to thin it out as needed and minced garlic.
Once the mayo is thick, fold in your chopped herbs and set aside.
Spread a heaping tbsp of mayo onto each slice of bread.
On one slice of bread, smash one half of an avocado. Top that with a few leaves of lettuce and some slices of fresh mozzarella
On the other slice of bread, layer on some of the sprouts and then top with thick slices of the heirloom tomatoes. Season the tomatoes with a pinch of salt.
Sandwich both slices of bread together, repeat with the remaining slices of bread, and serve immediately.