Cook the Bacon: Heat a skillet over medium heat and arrange strips of bacon. Cook for 5-7 minutes until crispy, then transfer to a paper towel-lined plate.

Scramble the Eggs: Whisk together eggs, milk, salt, and pepper. Pour into the warmed skillet and stir for 3-4 minutes until slightly underdone.

Slice the Croissants: Carefully slice the croissants in half horizontally and place the bottom halves on a baking tray.

Assemble the Sandwiches: Layer scrambled eggs, bacon, cheese, tomato, and spinach onto each croissant half. Add mayonnaise or mustard if desired.

Warm the Sandwiches: Preheat oven to 180°C (350°F), place sandwiches on a baking sheet, and warm for about 5 minutes until cheese is melted.

Serve and Enjoy: Remove from oven, place the top halves onto each sandwich, and serve immediately.