In a small bowl, mix the mayo, lemon juice, and a generous amount of cracked black pepper until well blended.
After preparing and slicing your veggies and cheese, toast your slices of bread.
Once toasted, spread half of the mayo on one slice of toast.
Add the cheese as the bottom layer, then add the tomatoes. Add the avocado and spread it out over the tomatoes--almost like a fan as seen in the photos. Then, add the red onion.
Spread the remaining half of the mayo on the other slice of toast.
Sprinkle the sprouts over the mayo (this helps them stick and keeps them from tumbling all over the place).
Top the sandwich off with that last piece of bread, cut if you wish (slowly and carefully to avoid squeezing all of the contents of the sandwich out from the sides), and enjoy that bad boy!