Prepare the avocado slices by fanning each avocado half out on a cutting board and sprinkling them with salt and pepper. Set aside for later.
Spread the butter out on each side of the sourdough bread.
Heat a large skillet over medium heat and spray with nonstick cooking spray. Then, add the sourdough bread to the pan.
Let the bread toast for 2-3 minutes or until golden brown.
Flip the pieces of bread around so the toasted side is facing upward and place two half pieces of provolone cheese on top of two of the pieces of toasted bread.
Sprinkle a few red onions on top of the cheese and fan out ½ an avocado on each piece of bread.
Sprinkle the rest of the red onions over the avocado and top the avocado with two more half slices of provolone cheese.
Top each sandwich with the other piece of bread (toasted side down) and toast the grilled cheese until the provolone cheese begins to melt (about 2-3 minutes).
Carefully flip the grilled cheeses over and toast the other side of the grilled cheese sandwiches.
Remove from heat and serve with an option of a salsa dipping sauce.