01 - Preheat oven to 350°F.
02 - Slice croissants in half horizontally and set aside.
03 - In skillet over medium heat, cook bacon until crisp. Drain on paper towels.
04 - Whisk together eggs, milk, salt, and pepper in bowl.
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 thin layer of mayonnaise or Dijon mustard inside each croissant if desired.
07 - Layer each bottom half with scrambled eggs, 2 bacon slices, 1 cheese slice, tomato, and spinach. Top with avocado if desired.
08 - Place sandwich tops on and arrange on baking sheet. Warm in oven for 5 minutes until cheese melts and croissants are heated through.
09 - Serve hot.