Begin by cooking the bacon in a skillet over medium heat until crispy. Remove and place on paper towels to drain excess grease.
In the same skillet, reduce the heat to low and toast the bread slices until golden brown on both sides. Set aside.
In a bowl, whisk the eggs and season with salt and pepper. Pour the eggs into the skillet and scramble until fully cooked.
Spread mayonnaise and Dijon mustard on one side of each slice of toasted bread.
On four slices of bread, layer the cooked bacon, scrambled eggs, fresh spinach, tomato slices, and avocado.
Top each sandwich with the remaining slices of bread, mayonnaise side down.
Heat a little butter in the skillet over medium heat and grill each sandwich for 2-3 minutes on each side, or until golden brown and heated through.
Remove from the skillet, slice in half, and serve immediately.