01 - Preheat the oven to 350°F.
02 - Slice the croissants in half horizontally and set aside.
03 - In a skillet over medium heat, cook the bacon until crisp. Drain on paper towels.
04 - In a bowl, whisk together eggs, milk, salt, and pepper until well combined.
05 - Melt butter in a nonstick skillet over medium-low heat. Pour in the eggs and cook, stirring gently, until just set and soft. Remove from heat.
06 - If desired, spread a thin layer of mayonnaise or Dijon mustard inside each croissant.
07 - Layer each bottom half of croissant with scrambled eggs, 2 slices of bacon, 1 slice of cheese, tomato, and spinach. Top with avocado slices if desired.
08 - Place sandwich tops on and arrange the filled croissants on a baking sheet. Warm in the oven for 5 minutes, or until the cheese is melted and the croissants are heated through.
09 - Serve hot and enjoy!