In a small bowl, mix together the mayonnaise and Dijon mustard. Set aside.

In a skillet over medium heat, cook the bacon until crispy. Remove and place on paper towels to drain.

In the same skillet, crack the eggs and cook to your preference (sunny-side up, over-easy, or scrambled). Season with salt and pepper.

While the eggs are cooking, butter one side of each slice of bread.

In a separate skillet or griddle, toast the bread buttered side down until golden brown.

To assemble the sandwiches, spread the mayonnaise mixture on one side of each slice of toasted bread.

Layer the spinach, tomato, avocado, bacon, and cooked eggs on four slices.

Top with the remaining slices of bread, mayonnaise side down.

Cut each sandwich in half and serve immediately.