01 - Preheat oven to 350°F.
02 - Slice croissants in half horizontally and set aside.
03 - Cook bacon in skillet over medium heat until crisp. Drain on paper towels.
04 - Whisk eggs, milk, salt, and pepper in a bowl until combined.
05 - Melt butter in nonstick skillet over medium-low heat. Pour in eggs and cook, stirring gently, until just set and soft. Remove from heat.
06 - Spread mayonnaise or Dijon mustard inside each croissant half if desired.
07 - Layer bottom croissant halves with scrambled eggs, 2 bacon slices, 1 cheese slice, tomato, and spinach. Add avocado if using.
08 - Place sandwich tops on and arrange filled croissants on baking sheet.
09 - Bake for 5 minutes or until cheese melts and croissants are heated through.
10 - Serve immediately while hot.